I've done that sort of thing a few times. Here's a couple of 20's over a 55;

Here's a rack with 2 29's next to a rack with 3 10's;

This one has a couple of 20's on the bottom front, a 20 & 30 top rear, with room for an old 63 gallon above the 20's;

I would put the pair of 29's on the bottom, with a pair of legs in the middle for support. The 10's acrooss the top will bow a little, but not enough to matter. The pair of 20's in the first pic has around 3/16" deflection in the middle, doesn't harm a thing. I've seen fishrooms with 5 or 6 10's across the short way, with better than 1/2" deflection, and they hold up. When you build the rack, leave enough room to give a decent siphon. That bottom 10 & the bottom pair of 20's take some tricks to siphon.
I switch fish, tanks, & filters on a regular basis. When I upgraded a few 20's to 29's, I put the fish in a 5 gallon bucket with a sponge filter & heater. Drained the old tanks, filled the new ones, 100% new water. Kept my filter media wet through the whole process, added fish & filters to the larger tanks. These were all bare bottom tanks, substrate & decos are not an issue.
HTH, any more questions ask away!