2 base layers vs 1 thicker layer.

I think versatility wins.

Cheers

I would also go for versaltility. The thinner the baselayer, the less moisture it will hold and so the less heat will be drawn away.

In our colder months (SE) I generally go with a long sleeve Cap 2 crew, then either the Cap 4 long sleeve 1/4 zip hooded shirt, or my R1, and then my puffy. This has been a really nice system for me and it just works.

Some garments, mainly lower base layers, fit too snugly to be used together comfortably. I never wear more than one lower base layer.

If it's all the same material (e. g. wool, or in this case, fleece) than you might as well carry two layers. It is more versatile, and maybe a bit warmer (the air trapped between the layers adds insulation).

It gets trickier when you talk about puffy clothes. Puffy clothes have a puffy insulation (down or synthetic) surrounded by two layers of material (typically nylon). As you add insulation, you don't need to add much extra nylon to surround it. That's why a really think puffy jacket is more efficient (provides more warmth per weight) over a thin puffy jacket.

Then you have the fact that puffy jackets, by and large, provide more warmth per weight than base layer clothing (straight fleece or wool). I can keep adding fleece layers and be plenty warm, but will carry a lot more weight than if I just carry a single, big, puffy jacket.

This essentially leads to a trade-off: versatility versus weight savings. For me, personally, when I backpack, I carry one synthetic puffy jacket, with a synthetic T-Shirt. Other than a rain jacket, that is it. That is plenty warm for the weight, but not very flexible. It doesn't breathe that well, and I'm often in an in between temperature — a bit too cold to go with just the T-Shirt, and a bit too warm to go with the puffy. That is the price I pay to keep my pack weight down.

But for day hiking, I carry fleece instead. Flexibility is more important. The fleece has a bigger comfort range. As we get into shoulder season, I add the puffy to the fleece.

Well, with two layers, you will find that they are warmer (given equivalent thickness.)
They will also trap a bit more moisture (perspiration) than a single layer.

The first layer should be a smaller size than the next layer. This will keep the bulk down and allow more room in your pants/jacket.

The woven material (like smartwool and the like) under a second layer will tend to trap more air between the layers. This dead air space becomes an insulator as well as the material. If it is too warm, then you perspire under them, though. This will negate any warmth. It depends on how cold it is. I often carry 2 layers of light "long johns" with me rather than one thicker layer in October(~10-20F) in the ADK's. It simply does a better job and dries quicker once I stop to camp. The elastic can be anoying, especially with a new set. Stretch these a bit before taking them out. I often wrap them around a chair with an old pack in it. This will cut down on the "bite" that new elastic can give you.

I've found that in subfreezing temperatures, that it's better for me to wear the thickest baselayer I'd feel comfortable hiking in and not overheat, and to have a puffy on standby for breaks and such.

This is highly personal but this is how I roll baselayer/insulation wise:

Above 30*: nylon/spandex blend t shirt and wrap in sleeping bag in needed. I'm typically fine to freezing with just a rain jacket as long as I'm moving.
20*-30*: silk weight baselayer with puffy on standby in pack
<20*: military polypros, fleece vest sometimes worn, and puffy for breaks.

For hunting and whatnot where I'm not moving as much, I bump up my insulation a notch.

The point of the base layer is not to add insulation, it is to remove moisture/sweat away from your body. Thinner layers do a better job of this and also dry faster. Your midlayer is your insulation to trap warmth. Combining the two into one (thick base layer) compromises moisture transfer, reduces insulation effectiveness (damp layers of ANY material do not insulate as well as dry layers) and reduces versatility.

If you are questioning how many layers you are bringing i would look at whether you need two insulation layers, perhaps leaving the Cap 4 or the puffy layer behind depending on conditions makes sense.

Its more flexible … Which allows you to minimize sweat

The r1/t3 is not really meant as a base layer by itself but rather a mid layer

The cap4/r1 on the other hand is meant as a next to skin layer (or with an exceptionally thin and snug top under)

;)

One thin layer

Base layer also absorbs sweat and body oils which can get in your insulation layer

I wear just base layer when it's warmer, and to protect from bugs and sun. I want one base layer to provide all requirements.

Base layer is heavy for the warmth, better to have slightly warmer insulation layer

And 2 mid weight insulation layers rather than 1 thick one.

Wicking base layers are heavy for their warmth, and warm fleecy mid layers are clammy next to your skin.

One or both can also supplement a mid weight puffy jacket in camp, stops, etc.