Under a windproof shell mitten, is an insulating liner mitten significantly warmer than an insulating liner glove made of the same material and thickness?
This is a different question from "Are mittens warmer than gloves?" because that question is about mittens vs. gloves that aren't inside a windproof shell mitten.
Some facts (I think):
When exposed to the atmosphere, mittens are warmer than gloves because a mitten has less surface area to conduct heat or release water vapor.
Wind strips away the layer of warm moist air from next to the skin, speeding up conduction and evaporative cooling.
Insulation works by trapping air (a very slow conductor) close to the skin and slowing down convection within that layer of air -- that's why the walls of buildings are lined with fiberglass batts instead of left empty.
Wind makes insulation less effective by forcing air through the gaps, causing convection.
Speculation based on that:
Liner mittens and gloves equally do the job of trapping air next to the skin and limiting convection. Inside the windproof shell mitten, either one works at maximal effectiveness. Because there's no material between the liner and the shell to prevent convection, and because the shell doesn't prevent conduction, we can assume the air inside the shell is about the same temperature as the air outside. But the shell does minimize evaporative cooling by minimizing the surface area that's exposed to the wind.
It seems the only difference between the liner mitten and the liner glove is that the glove has more surface area for conduction with the air inside the shell mitten. But is that conduction even significant? It's a confined environment inside the mitten, and the fingers of the liner glove are usually in contact.