I will agree that an undertank heater would be better. The UTHs help provide heat to their bellies, which is important for digestion and its how they would be getting their heat in nature. I have found that my BP likes to spend a lot of his time in the middle temps area. I use a UTH hooked up to a thermostat and a heat lamp that is hooked up to it as well. The thermostat shuts them off when the hot side gets too hot and then turns them back on when the temps have dropped. Ambient temps stay around 82. Severus likes to spend most of his time in the middle, between the cool side and hot side. OR he spends a lot of time "basking" in/on his pool. Granted, where I live, it's very humid and hot right now, so that may have something to do with it. But they will naturally set out to regulate their body temperatures if you provide the correct environment for them.
You shouldn't use the same thermostat for two different heat sources. You can only double up if it's two UTH, RHP,or a light.