New to backpacking: Why does my backpack already seem so light?

You are off to a good start. Sharing gear will make it simple to get out and enjoy the trinity alps and make weight less of an issue.

Many here are unable to find dependable partners, and won't go out without a complete solo kit. You have a dependable partner and should feel comfortable that you are traveling together on a trip you are committed to finishing together.

Your other option is to replace everything and be prepared to split up, go solo, and drive home in separate cars.

Your shelter and two sleeping bags are at least double the weight that many "Ultralight" backpackers here typically use. There's lot's of other items you'll need to include and weigh, such as backpacks, sleeping pads, jackets, any extra clothing, repair kit, first aid, etc. Many of these items can add significant weight. If you have these things already, you could list them here and get some feedback.

Working backwards from 20lbs total pack weight, for a three day trip you'll probably need roughly 4-6lbs of consumables, which means you'd need to get your base weight down to 14-16lbs, which is doable with some of the items you've already have listed, but you're kind of pushing it with how heavy your shelter/bags are.

That is not a complete list of everything you will be bringing and sharing items will help but when you add in everything the weight will go up, my complete pack solo for 3 days including food and fuel but not water comes in at between 11 and 12lbs depending on time of year and weather,how much does your pack alone weigh? Your sleeping bag weighs 3 1/2 pounds the other is just under 3 lbs(most people on bpl carry bags half that weight for the same temp. rating), your tent is almost 5 pounds (again most people here carry a 2 person tent that is at least half the weight of yours or go even lighter with a tarp). I am not saying you have to do everything or buy everything that people here have but you may be in for a surprise

General info for you to look at
Mike Clelland(NOLs instructor and author, his books are very good),he has some great free videos on lightening up be sure to watch(his clothing system,the entire contents of his pack,water treatment and part 1 and 2 on the dinky stuff for ideas),this is an article he wrote The fastest way to backpack weight loss ,this is pmags Lightweight Backpacking 101 and The Frugal Backpacker – The $300 Gear Challenge .These are some other articles and videos for you to check out

Backpacking for Cheap: Gear for the Gearless

Backpacking Checklist (Gear List): 3-Season, 3-Day

Lightweight Backpacking 101: An Introductory Manual for Lightening Your Load Today (1st Edition – August 2001)

Cheap Ultralight Gear List

Oregon Field Guide: Ultralight Hiking

Lightweight Testimony: Lighter, Farther, Faster
Jamie Shortt talks about his progression and shows his gear list for each stage, Lightweight Testimony: My Journey into Lightweight Backpacking
CleverHiker Light weight Basics
CleverHiker Trail Skills

Andrew Skurka has a very good website with trips and gear lists for you to check out, here is a talk he gave at google
Ultimate Hiking Gear & Skills Clinic
His book is worth checking out also
The Ultimate Hiker’s Gear Guide: Tools & Techniques to Hit the Trail
You can get little repackaging containers(like the ones you see in Mike Clelland uses in his videos) many places including your local stores and pharmacy but here are some examples
USPLASTICS
USPLASTICS
Ultralight Designs
GossamerGear
MountainLaurelDesigns
You could make toothpaste dots like Mike C. shows or there are Archtek Toothpaste Tablets so only bring exactly what you need in a snack size bag for any given trip instead of a whole tube and THESE are good for little thing like pills, toothpaste tablets ect. so you are not packing a whole bottle(you can also get them very cheap at most pharmacies) .

Yeah you dont have a lot listed like pads, First aid, lights, clothes is a big one that will creep up really fast. weigh your clothes too. how heavy is your pack? 5 lbs? say 4… x2 = 8lbs + 12.4 lbs from earlier theres your 20.

all the little things add up. you dont have water containment. (treatment, but what are you putting it in?)

I think you can get there or at least close. say you get close and just want to change one thing. Assuming you sleep comfortably in you SB, just change out your tent. One item you can drop like 3 lbs easy.

20 is pretty easy these days in my opinion, especially if youre splitting stuff

Thanks for the feedback. Great links as well, definitely good stuff in there which I'm going to have to pick through.

These items were purchased at REI:
-tent
-sleeping bags

and can be returned.

Half Dome 2 was purchased on the basis of positive feedback and the fact we could split it between the two of us. Also the lack of mesh at the base – I hate dust blowing into the tent, and outside summer most of our trips will be desert. After looking at so many tents it seemed the most versatile and reliable within our budget. It pains me to look at the weight, but for now I think we'll stick with it, unless convinced otherwise. Any tent suggestions?

The sleeping bags, however are something we could part with. According to the guide book we have, nighttime temps in the TA range from 25-45 degrees. And they met our budget of $150 or less per bag. Any suggestions here?

Backpacks are another intricate issue we haven't yet tackled, mainly because we don't know what our pack weights/space needs are at this time.

That's not bad, but your full gear list will be a LOT bigger than that.
The critical thing now is that you do NOT relax and start throwing in extra non-essentials 'because they don't weigh much'.

Cheers

Check out gear lists from from experienced folks here who hike and camp where you plan to hike and camp.

Read all of Link's links, and get a good scale… or a few good scales. I have a Kg scale accurate to 0.1g, a 10lb postal scale accurate to 0.1oz and a decent digital 50lb fish scale.

All the "dinky stuff" adds up and a true accounting of how much your pack weighs can't be made until you have accurate weights for every single thing you wear and carry on your back, in your pockets, and in your hands.

You'll come to appreciate the meanings of "base weight", "consumables", "total pack weight" (TPW) and "full skin-out" (FSO).

Looks pretty good so far. If you can keep your total pack weight under 30 pounds then you should be comfortable.
I wouldn't stress out too much about buying a bunch of fancy new gear right now, just use what you have for a few trips and you will get an idea of your own gear preferences.
If you both decide you are serious about backpacking and want to get a good kit, I highly recommend the "buy once cry once" approach. Buy high quality stuff first instead of buying medium quality stuff that you will upgrade later, which costs you a lot more overall.

The half dome 2 is ok if you are planning on sharing the weight, but if you plan on doing anything solo I would get a lighter tent. Pyramid shelters are light and provide a lot of protection.

The Trinity Alps will be hot in August, you can pack light for clothes, don't really need a big down jacket or anything like that.

Yeah you're missing a few things on there. Backpack especially figures into baseweight.

I pimp this a lot lately but Skurka has done a great starting point for a clothing system that is modular enough to handle 3 seasons almost anywhere by utilizing about half of it any given time.

http://andrewskurka.com/2015/backpacking-core-clothing-thirteen-3-season-conditions/

Now camping in one climate means you can customize your wardrobe somewhat and cut down on gear and weight.

But to be fair it's one of the first really blunt articles to go into a modular clothing system and the thought behind it. It helped me shift a few gears and lighten up my pack.

Do you use trekking poles?

If you do, get a shelter that uses them.

(e.g. TarpTenet, and there's options for the inner on the Stratospire2, you can get solid at the bottom)

My initial thought on reading the title was that you are still in your 20's or 30's!

Yes, several things I don't see on your list: 1st aid/emergency kit, bear bag or critter proofing, rain gear (I know there's a drought, maybe it never rains?), and camera/phone. Unless I missed it, you also don't say what packs you are carrying. Pack weight figures in, too. I think it's pretty easy to shave pounds and gets pretty expensive to shave ounces. Good suggestions here.

a. Probably 4lbs of water each.
b. The clothes will be 5lbs if you are not careful. Bring no extra clothes.
c. Gotta include the weight of your backpack, say 5lbs
d. Raingear? say 3lbs
e. water filter? say 1lb (each should carry)
f. first aid kit say 1lb (each should carry)
g. two little flashlights (too light to matter)
h. maps/guidebooks? maybe 1lb

So that adds 20lbs.

a. Probably 4lbs of water each.
b. The clothes will be 5lbs if you are not careful. Bring no extra clothes.
c. Gotta include the weight of your backpack, say 5lbs
d. Raingear? say 3lbs
e. water filter? say 1lb (each should carry)
f. first aid kit say 1lb (each should carry)
g. two little flashlights (too light to matter)
h. maps/guidebooks? maybe 1lb

So that adds 20lbs.

Umm… This is a joke, right?

You only need 1 foodbag/bear canister, first aid kit, kitchen(share 1 pan,1spoon,1windscreen , 1stove), water treatment, camera, shelter, knife, trowel, soap, sunscreen, map, compass.

If you are more into hike than camp, you can go with 1 puffy jacket / warm pants and take turns.

If you don't hike after dark, you can use 1 lamp. working together you can climb and descend peaks with a shared lamp, staying on trail shouldn't be that hard.

After reading the list, I loved to imagine that was all they would take.
leaves for toilet paper. Dirt for toothpaste. Rubbing sticks together for fire
Tom Hanks in Castaway showed us what ultralighting was all about.

"a. Probably 4lbs of water each.
b. The clothes will be 5lbs if you are not careful. Bring no extra clothes.
c. Gotta include the weight of your backpack, say 5lbs
d. Raingear? say 3lbs
e. water filter? say 1lb (each should carry)
f. first aid kit say 1lb (each should carry)
g. two little flashlights (too light to matter)
h. maps/guidebooks? maybe 1lb

So that adds 20lbs.

Umm… This is a joke, right?"

That's actually what noobs who show up at my backpacking class bring. Happily, the OP has come to the right place, and the noobs go on the class trip with substantially less than that.

Yeah sounds like it may not be as light as you thought. Don't worry that's pretty much what happens to everyone.

Don't let the shock discourage you and spend another couple months gathering the gear. Just get what you need and go man. I've backpacked with a full comforter strapped to my backpack for a friend.

"refining" your gear is more of a constant than an initial step. A quarter dome is a great tent. When you want to replace it, look to a Cuben fiber Tarp (4oz) and a 6 oz bug net. They're actually relatively cheap compared to a tent, but REI won't have them.

Does this sound like a good starting point to make it under 20 pounds each or am I fooling myself?

If you can manage it, find something really close to do a couple separate overnight 'shake down' trips. This can really help in figuring out if there is some areas of technique that need researching, and if there is gear that isn't really needed or something that is missing.

Research and practice blister prevention and treatment. Even if you don't experience them personally, other people can cut a trip short or really slow you down if they get some bad ones and nobody knows much more than moleskin. There are people I've day hiked 20 miles with on a Saturdays while they were wearing cotton socks and sweaty Gortex boots, and they have no blister problems. Then they toss a backpack on for the first time and I'll go through half my first aid kit keeping them walking.

As mentioned above, you need a full HONEST gear list to see where you are are. You have listed some of the classic "big 3" items, but there is more to an effective and safe kit.

You can check the community gear lists at http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/user_gear_list_index.html for samples.

Below is a generic gear list for my typical 3-season ultralight kit. Climate, temperature range and personal metabolism should be considered with the clothing list, which is deliberately minimal. It is assumed that any liquids like bug repellent or sun screen would be decanted to the smallest possible container for the trip.

Of course, the devil is in the details:

Weigh everything and make a list (most use spreadsheets).

Seek out those products that deliver the highest performance, lightest weight, and multiple uses.

If you are not going to use a particular item, leave it at home. Keep electronic toys and other extras to a minimum. Avoid spares and redundant items.

Dirt will not kill you– you don't need clean clothes for each day. ONE spare pair of socks is enough.

Be diligent and you will be rewarded with a comfortable, safe, and light kit.

The list:
Backpack
Pack liner
Trekking poles
Shelter
Ground cloth
Guy lines
Stakes
Sleeping bag
Sleeping pad
Knife
Headlamp
First aid kit
Whistle
Compass
Map(s)
Fire starter
Sunglasses
Insect repellent
Sunscreen
Small repair kit (duct tape, sewing kit)
Water container(s)
Water treatment
Cook pot
Stove
Windscreen
Fuel container
Spork
Cup
Food
Bear bag
Line
Shoes
Socks
Briefs
Base layer shirt
Pants/shorts
Insulation layer
Windshirt
Bandana
Insect head net
Hat
Beanie
Gloves
Rain jacket/poncho
Rain pants
Potty trowel
Toilet paper
Hand cleaner
Soap
Toothbrush
Floss
Toothpaste
Comb

The only thing I will add is eating 2 mountain house meals per day isnt that normal or healthy. One for dinner is fine, but for breakfast all you need is some oatmeal and precooked bacon.

One great reccomendation for lunch is tortillas, dry salami and a knife, and your choice of cheese. All of these last several days in the heat, not direct sunlight or extreme heat of course. But it tastes really good to me.