Two thin layers vs. one thicker layer?

[Sodium]

I'm thinking about long sleeve base layers for sitting around camp on cooler evenings. Obviously with an insulated jacket on top, but as far as base layers go, would two 90g (3oz) base layers worn together be warmer than, say, a single 225g (8oz) base layer?

For short trips when I've got a better idea of what the temperature is going to be like, I've always just gone for a single thicker baselayer. But two thin layers would be more flexible on a thru-hike, and perhaps warmer too? Because of air trapped between the layers?

Thanks

[Connie]

This is why I have 200 wt merino and a less than 2 oz. wind shirt.

I have a MontBell UL Thermowrap vest and/or a rain shell if I need more.

These layers get me thru 3-seasons, except for really hot weather. Then, it is a rayon shirt or a nylon "fisherman's" vented big shirt over a skimpy undershirt so the outer shirt does not get all "sticky" and uncomfortable.

If you are going for warmer layers, 260 wt merino, artificial insulation, and down insulation for inactivity.

[Havana]

I don't see the sense in packing two layers of the same weight. I generally have multiple layers of increasing weights. So I usually wear a Cap1 weight t-shirt, will layer a 100/200wt fleece on top of that, then depending on the weather the next layer could be either a heavier weight Capilene/Fleece or something like Patagonia Puff/Down Sweater, final layer is a rain/wind jacket. Mix and match until you're comfy. So, a long way of saying, I'd have two layers but of different weights.

[Just Bill]

Multiple thin layers generally are warmer.
In your OP scenario the two pieces would be better than the one. If nothing else multiple light layers give you better control of your temps.

That said I would echo the others in suggesting multiple light layers of different types.
A combo I like lately along your line of thought is a merino 1 tank top with a merino 1 ls shirt.
But generally I would add a button up (sun shirt) type and wind shell to compliment the base layer to get started.

Remember the basic 1,2,3 system of base, insulation, shell and you'll do pretty well. This works even in lighter layers where you could call a tank top a base, a 90g top insulation, and a wind shirt/shell your shell layer.
I would personally rather have 4-5 lighter pieces but some do find success with a base top, puffy, and windshell too.
Base when hiking, base plus wind when on break or above treeline, and all three on at camp.

No matter how you do it- verify that the layers actually layer. Two of the exact 90g top will not work that well. But a medium and large (one size up) would work for example. Same with your shell in combo with a puffy- buy it to fit big enough so that when worn together your shell doesn't compress the loft of your insulation layer.

Generally Patagonia clothes are cut this way to start, so in their case you may wear the same size in base, insulation, and shell pieces.