Do you already have the 2 20's. Reason I ask is it's gonna be dadgum tight with those. I calculate back wash at 17 gallons. Probably gonna be a little less than that, but we'll be pushing our luck. I like to leave a little wiggle room. 2 29's would work much better. or even a 29 and a 20 would work. Might have to elevate the 20 a couple inches, might not have to. A standard 29 is 30X12X18. I know I've bout mine for 60 bucks apiece. Anyway, I'll design it whatever way you want to go.
One return pump, or two? one will give more room in the sump/fuge, two will let you just run on one if you need to for maintanence or a failure. Of course two will cost a little more than one will. But just about 30 bucks more....ish. Do you want to run some other stuff off the return line? Reactors, U/V etc? If so, what? What about water change valve?