I added my second brood boxes on 2 hives underneath the existing boxes this year. It worked very well, and I will probably always do it this way now. I did not move any frames from one box to the other. The benefits of adding a new box below are:
- Maintenance of brood warmth in the existing box (heat rises and bees try hard to keep the brood temp stable at 35C)
- Bees naturally like to build downwards. If they can't do that, they like to maintain an "expanding edge" of comb. If your new empty frames are below, you will get much stronger, more orderly comb. If they are above, and you have foundationless frames, you may end up with some very creative comb which is wasteful of bee effort and undesirably messy.
I am not the first, nor the only person to add new brood boxes below the first one. In fact, Warre hives have done it this way for a long time, very successfully. The only disadvantage that I have found is that you have to lift a heavy brood box to put the new one underneath. My bees seem to like it, and I do too!