I made my bin out of a plastic bin much like yours Matt and have had trouble coming up with a way to separate the worms from the dirt, I love your idea of stacking two bins together. I will purchase another bin and put your idea to use. I have two bins going and when one is full I move over to the other one, letting the worms finish processing the full one. I do however, lose worms with that system. I separate what I can but still end up losing quite a few.