Two screens usually separated by about 3/4". You can put a colony on each side of the screen. Generally the bottom box can't get out the top, but the top box can get out the bottom. If you're stacking colonies, it's common practice to put the entrances on opposite sides.
Edit: That'll teach me to not read carefully. I thought you were talking about double-screened _bottom boards_. Ignore that whole bit about entrances.
Edit: That'll teach me to not read carefully. I thought you were talking about double-screened _bottom boards_. Ignore that whole bit about entrances.
