I had a similar problem. I have a 55g freshwater that came with a stand, I got a second 55g to set up as a saltwater but it was just the tank with no stand. The stand for the freshwater is not being used but feels way to flimsy and rocks with an empty tank sitting on it. I bought a 29g for a sump/fuge combo but there is not enough room in the stand for that size tank.
You have a couple options:
A)buy a bigger stand
B) build a bigger stand (the option I am doing)
C) buy a custom sump to fit your stand.
D) make your own out of acrylic
I was also having some issues with using a canister pump along with a sump, but much like you biff showed me how my setup had flaws that could lead to gallons ending up on my floor......bad juju there!
One thing you could do if you wanted to stick with 2- 10 gallon tanks, is have one overflow into the first, then flow into the second, then a return pump up to the display tank. You would have to set it up to make sure if your pump fails that the overflows stop flowing, there are a few videos on you tube where people have a number of sump and refuge tanks all set up with only one return pump.