Yeah you can do one on top of the other too, play about with how to divide it up and see what works best. Just take into acount the height of the pots, the minimum space your lights will need etc. If you go for having one on the top and one on the bottom make sure the shelf that your gonna put the flowering plants on is plenty strong enuff, the pots get pretty heavy when you've watered them or have the veg room on the top! Just so you no i'm not very experienced, i've a couple of grows under my belt but i don't claim to no loads, just giving you my opinion.

:rasta:

Remember they'll double or triple in size from when you start to flower, so consider that when you decide how to divide your space