I will go with MSD critic, but beyond that, I want one or three! My rabbits love (some old chinchilla cages that have) 2nd floors.
I have pictures the cages, in my gallery. I have one odd idea though and if they do it, please post. Would they eat hay on the second floor and potty on those below them, or do they already have a potty area? I have not had enough nerve to do it on that size cage, I did not know if they wire would be strong enough, or would bow under the rabbits weight. I would be one way of getting more floor space, out of one 2ft tall cage! Looks nice!