So are you saying that you have two UTHs? One on each side? And did you say you only have one hide?
It could be that his tank is too warm or he wants to feel secure on the other side of the tank. Could you snap a picture of your enclosure and tell us what the temps are on the hot side, cold side, and ambient temps? Can you tell us what you keep your house at? This way we can give you some advice on how to best set up your enclosure to make him feel more secure and comfortable.
Also, does your UTH (s) have a thermostat to regulate them? If not, they can get upwards of 130* if you let them and that can cook your snake...literally. You might want to purchase one if you don't have one already.