Removing RAM from an old laptop and installing it in another?

I have two old laptops at home, one has two slots for RAM with 2 x 2GB, the other only has one slot with 4GB of RAM. Now I wanted to ask whether I could remove the RAM from the one with just one slot and simply install it in the other so that it would have 6 GB of RAM

You can try it, but it doesn't have to work.

Theoretically yes, but there are different RAMs

DDR1, DDR2, DDR3, DDR4

I think 1 and 2 will fall away.

If the two devices have the same memory types, you can easily combine them