K, I'm sorry for coming off like that. This thread is just 5 pages deep and thats a lot for this type of thing but w.e lets get you the help you need.
A rheostat WILL control 2 heat sources but its only effective when its controlling two of the same. A thermostat will do the same. See when your using two different heat sources like wattages, they heat up at different rates, so thats the problem your running into. When you use a thermostat it uses a probe that measures the heat and shuts it off once its there. You would use this for the UTH. The infrared should be fine for ambient temps, but you might need two like me. I used a smaller wattage light for the cooler side like I said before. If one light is doing the trick but is still getting too hot then you could use the dimmer for the light, and a thermostat for the UTH. Petco/Petsmart now carries thermostats that work good for your application for 50 bucks and can handle up to 1000watts. Thermo's really are they way to go and are going to save you a lot of headaches, and burns for your snake.