actually I just had a thought - could you not get something like a tank stacker? there are definitely stands made for aquariums that have two levels or more, so that you can have more than one tank without taking up more floorspace. I'm sure they must have a big enough gap between the levels as well, as otherwise how would you change water/add/remove fish? especially with the weight of water, they wouldn't expect you to take the tanks off the stand to drain or refill! let me see if I can find a picture to show what I mean...
like this!:
if you or your OH are quite handy, you could always make your own. in searching for examples, I found loads of DIY tank stacking stand instructions.