Well, when I split the hive in early spring, I plan to put the split on top of the main hive (facing a diff. direction of course) until I recombine in the fall. The purpose is that I have very limited lateral space in which to place a second hive. stacking them like that will allow me to keep the second hive without trying to find space for it on my property. Of course, as long as I'm keeping the Italians, I'll have to separate them to try and prevent robbing out, but once I make the change to carnolians, I want to stack them vertically.