It is almost universally recommended that wearing multiple layers of clothing is the most effective way to stay warm in cold weather, compared to one thicker layer, although there seems to be little actual science to support this claim. e.g. "The reason wearing multiple thin layers will keep you warmer than a single thicker layer is because warm air is trapped between the layers acting as an insulator" (here), or via this video here.
The rationale is that multiple layers trap more insulating air, thus acting as a barrier to heat loss. I find this argument doesn't hold up logically: if a double thickness layer of the same material is the same distance away from the skin as two thinner layers, then the volume of air trapped would be the same. I suspect that one thicker layer may also be better at forming a barrier against the wind and letting warm air escape. I also note that this 2010 study comparing the clothing worn by Scott and Amundson in their 1911-12 expeditions to the South Pole found that the Amundson team's thick fur coats were more effective than the multiple, thinner layers worn by Scott, despite being roughly the same mass. (Clearly, these are made of different materials so does not directly answer the question.)
Multiple layers have the advantage of temperature control, allowing you to add or remove layers depending on the conditions (as is articulated here) but I am not trying to find a practical advice for hiking/outdoor pursuits, rather look to find out whether - scientifically speaking - two layers are better at insulating heat than one, assuming material is constant and total thickness is the same.
I wonder whether there is any evidence or scientific (e.g. physics) arguments to settle this basic heat conservation question?