Id do as you said, 40 gal for Carpet which is long and higher for something to crawl on and the 20 long for the Ball...
I like the 30 and 33 Longs. 48x13x13
If you can go 48" L for both the snake enclosures that would be perfect to grow into. Keep a eye on Craigslist for a one and mybe sell one of your is you dont need it after.