If they are the same height, you could make an L shaped tank with a bulkhead between the 2.
I was actually thinking about this the other day, but it would prolly work better with larger tanks...
If you put 2 right next to eachother and had one be a predatory tank and 1 be a reef and just have rocks cascading from high on the reef side to low on the fowlr tank, but scape it in such a way that it looks like the rocks are continuous. (if that makes sense)
You could make a 30 gallon and use the 20 as a sump. Or like musho said, make them into QT tanks (always useful), frag tanks, or save em for a new project if you want.