You need to find out what the temps are in the tubs. The snakes should remain separated unless you are putting them together for intentional breeding purposes. And even then, it is a short-term situation. Once you have something to accurately measure the temps inside the tubs, we can give you more advice on tweaking your setups. You should be able to use just the UTH to maintain temps in a tub. Do you have a thermostat or a rheostat/dimmer regulating the UTH on each tub?