What degree sleeping bag do you need?

I would say it depends on the bag.

Some are over rated like the marmot pounder, Lafuma etc, Some are dead on like Montbell and others.

As far as taking a 40dF bag to 30dF with no layers, I doubt it, especially if its an over rated bag like a pounder.
That would be like taking a real rating of 50dF down to 30dF.

IF you get an honest rated bag, like a montbell, WM or other, then 30dF should be doable with some extra clothes.

That said, last spring I slept in a Montbell #5 bag which weighs 1#2oz and is rated to 40dF. It was an honest 40dF out, I was outside, no bivy, heavy moisture in the air and I slept very warm.

That said if you go for a Montbell, get the #4 as the #5 does not have a zipper baffle. Better yet the 30dF #2 has a neck collar, if you expect to be in colder weather more.

I'm considering the Big Agnes Yampa 40° Sleeping Bag (650 Down). They use 9.5 oz of 650 down.

I know these aren't necessarily favorites here, but I love BA bags (I have an Encampment now and love everything about it but how much it weights).

I think you would gain a lot of warmth by zipping the bags together. A double quilt would probably work even better, or get someone to make you single quilts that zip together. And where I live, a 40 degree bag is overkill most of the year.

“Finally, some couples like to use two person quilts such as the Nunatak backcountry blanket. If a couple sleep close together, say spooned, they should be comfortable in temperatures which are 10-15F colder than they would be comfortable in under the same insulation without someone else”

http://www.verber.com/mark/outdoors/gear/sleep-system.html

I think that is pretty accurate.
Have fun!
Jay

In my experience zipping two bags together drastically lessens their warmth, you'll be warmer sleeping separately in zipped up bags. The air space is exponentially increased, and the draft collars and hood closures cease to be effective.

Theoretically, a two person bag should be lighter and warmer due to shared body heat. This requires a purpose made double bag/quilt, and (for any sort of real cold) double hoods with a separator between each. Making one for my wife and I is on my list.

I have found the same to be true. They don't seal up as well and I have found it to be drafty with two people with two bags zipped together. Now three people in two bags zipped together might be warmer due to the decrease in air and drafts and increase in body heat. However that is something I haven't and won't find the opportunity to test.

For what it's worth, for most general camping, my girlfriend and I have a pair of Exped Wallcreeper bags… 750-fill down, and the bags are a very different design from anything else on the market. They have a full zip down the front, zippable armholes, cinchable footbox, and you can walk around camp in them, using them as long, down overcoats — fantastic for cold nights.

They zip together well, and while rated at 30 or 35 degrees for the model we have (ours say 35, but I think they upgraded the rating to 30 on the current model, which appears identical and weighs the same), they do fine at a variety of temps… they zip together well, and the various armholes and footbox openings mean that there are plenty of places to vent heat on one side or the other if the two sleepers are at different temps or if you are sleeping in temps well over the bags rating.

I've slept solo in mine down to below freezing and was warm and happy in a thin shirt, ultrathin silk pajama pants, wool socks, and a light beanie; albeit on top of a Eco Thermo 6 inflatable pad, which does a great job insulating from the ground.

The bags we've got weigh 31.7 oz each, and the perks are that they just seem really well designed. They come with well-made waterproof stuff sacks (1.9 oz), which is a nice feature, and the loose storage bags (2.8 oz) they come with are made of a fine, essentially noseum mesh, which you can take camping with you and wear over your head as a mosquito screen. It's not rocket science, but it's a nice attention to detail and speaks of a quality product.

I don't know if anyone else here uses them or not… it seems like no one who posts regularly uses anything less than 800-fill down bags. But I consider myself a lightweight backpacker who is in-transition to an ultralight pack. And right now, for most of the camping I'll be doing, the Wallcreepers appear to be one of the best, most versatile options on the market, and with a minimal weight penalty.

I wish you well in your search.

Trying to figure out where the extra pair of legs would go.

Larry,
A 20×20 "draft-stopper", like a chunk of fleece, or a "down curtain" sewn to the center of the top bag, parallel to the edge, will go a long way to increasing comfort. Otherwise, every time one of you rolls over the "bellows effect" will create a chilly air exchange.

We've been bping together for eons and we've tried it both ways. Zipped together just never worked. Too much cold air enters, partner's movements are disruptive, etc., etc.
They call tandem kayaks divorce boats…..I think that they should call doubled bags divorce bags.
Just my .02.

I tried the 2 bag zipped things, we use to have 2 Valandré classic 700.

It was either too hot for me or too cold for her; and the added warmth from being two was more than lost to more drafts, she was able to sleep in colder temp being alone.

I would agree that the typical two bags zipped together doesn't typically give a warmth advantage… my experience is like others I think it's worse because you end up with a bunch of empty space that billows when someone moves. Quilts, and I would guess purpose built dual bag,s can work well provide the couple sleeps well together.

–Mark

>>…bags can work well provide the couple sleeps well together.<<

I think this is the key. My girlfriend and I usually bring only one sleeping bag in the summer months. We use it as a quilt.

With a prior girlfriend, I could never make this work. We just didn't "sleep well together".

I had to get a new girlfriend, in the interest of UL.

"Quilts, and I would guess purpose built dual bag,s can work well provide the couple sleeps well together."

We sleep well together (a lot of practice) but even a double quilt doesn't work very well. We bought a nice JRB double quilt that we thought would be great but now we only use it over both of our 'separate' bags for winter use.
YMMV, of course. ;-)

Most of our trips will be summer trips, so zipped together should be fine. We've slept that way once and had no problems, but it certainly was NOT cold. ;-)

So, 40 degree bags okay? I've considered a double quilt, but I'm really unsure. The kit from thru-hiker is tempting…

You can always unzip a bag to ventilate if you get cold, but if you get cold in the bag all you can do is start piling on layers. I would error toward the warmer bag. As people have mentioned, ratings can vary widely. Most bags you need to add 10-15*F to get the "comfort" rating. Others, like WM, are generally comfortable at their rating.

It sounds like you two sleep around the same temp; if not, or for those of you who don't, I used to zip together different-rated bags. 0*F weather? She'd sleep in the -40*F bag, I'd sleep in the 0*F bag… worked out great for us. So well, in fact, that I've thought of making a down quilt for home with different amounts of loft on each half.

The double top bag I'll eventually make will have half zips on each side for the reason stated above. I'm usually much warmer than my wife. The whole point of a double bag is to take advantage of that.