I would probably buy a plastic tub for the ball python if you can't afford a new tank right now, and keep the corn where it is. Getting a tub for the ball would be a cheap and easy solution to getting humidity up, and even if you would rather house her in a tank for display purposes, a tub will provide her with better conditions until you can afford one. Honestly, those screen-type enclosures aren't the best for most types of snake, they're meant for chameleons which require a lot of ventilation to thrive. If you don't go for the tub, I would at least switch the snakes and keep the ball in the 20 gallon with most of the screen top covered with saran wrap to keep humidity in, and just use the heat pad, not the light- you really don't need both and the light will suck the humidity right out.