I haven't kept two at once, but I know several folks on here do and will probably chime in. I've seen several very creative setups on here. I like the double decker front opening setups. They seem relatively space saving and could probably be built into a large dresser or tall piece of old furniture. You could also try simply stacking two crossfires on top of eachother with some kind of cubby or spacer in between them for storage or running wires etc.
I wish I was more handy! I saw the crossfire set ups and a double decker would be great I am just all thumbs lol! And figuring out how to set up lighting in the tank has me baffled too. I can't wait to see what people have done though!!