Your best bet is to gently explain that having them together is bad for their health, and that moving one to a tub would be much easier and less expensive than getting another tank.
Also explain that while snakes she thinks a tub is too easy to escape, tanks are no different. Show her how you plan to keep the snake in the tub, perhaps a visual will help.
Ball pythons really do need a warm and cool side, however in a pinch if your mom just won't see reason, get yourself a piece of plexiglas and some clear waterproof silicone and divide the tank. Let each snake have it's own side for now. It may no be ideal, but it's better than the current situation. Depending on the size of your UTH's, you could have the outside edges as the warm spot, and the middle as the cool.
Humidity can be a real pain in a tank that size. My suggestions are: If you have a screen lid, cover about 90% of it with foil or even plexiglas. If you use heat lamps, try to figure out a way to do without them, they are humidity destroyers. If misting lightly is a option for you, try that. If nothing else works, try making humid hides by stuffing damp peat moss into a hide for them to use as they need it.
Gale