In the single blanket case, we have only a thick blanket that is acting as a barrier for the heat

from transmitting from the hot side to the cold side. Whereas on the double blanket side since we have thinner blankets, we also end up having a thin air packet gap between those two blankets, and as we know the air has a very low thermal conductivity value.

Since the thin packet of air has very low thermal conductivity it acts as an insulator preventing heat from transmitting across it. Hence the transmission of heat from the hot side to the cold side of one to the other blanket is very low when compared to the thick blanket.