I use the one I’m not covering myself with to place on top of the tent to trap my body heat in the tent or place it under the tent to insulate me from the cold ground? Which is warmer?
NEVER EVER sleep on bare ground or in a way there is no air insulation between you and the ground! You may never wake up again!
Ground is a terrible heat sink, and will suck your body heat. We were taught in the military to always isolate us off the ground with spruce twigs, grass, leaved and other stuff which creates a layer of air between the body and the ground.
Here is an illustration how it is done. “Laavu” means “bivouac” in Finnish. “Rakovalkea” is a fire set up between two horizontal logs – it will burn slowly and steadily, and emanate warmth.