If I want to separate units from a stack, I first select the unit within the stack so that it has the little up arrow on its icon, like this:
Then I right click on an adjacent tile, in this case the one southeast of the stack:
Doing this separates the unit, and If I repeat the operation and select another unit from the first stack and move it to the second stack, it joins the second stack.
This generally works well, except when you want to move units out of cities. In that case they don't always exit the city where you expect.
