Sounds good to me! Tubs are a bit easier to keep, but splitting a 20gL wouldn't be a bad idea.

Remember, lamps suck out humidity, and Ball Pythons don't require any special lighting. So, I wouldn't be too worried about having a lamp over the tank. If you do though, I would suggest putting aluminum foil over 70-80% of the tank lid so that it traps the humidity inside.