Question

why one thick blanket is less warmer than two thin blankets join together

Answer 1

Because there is a layer of air between them.

As strange as it may seem, air is a great thermal isolator, even better than most clothing.

The reason it seems it is not is because its movement evaporates water on the surface of our skin, thus diminishing the temperature, making us feel cooler.

The air between the thin blankets is not moving around and will form an isolating layer that acts exactly like another blanket, thus improving the thermal isolation.

Answer 2

One thick blanket is less warmer than two thin blankets join together because air cannot move in and out of two thin blankets but in one thick blanket air can move in and out . That's why one thick blanket is less warmer than two thin blankets.