No, you don't need both. You only need one or the other. The thermostat is the preferred option as it will either turn the UTH on and off to maintain your desired temp, or a proportional thermostat will limit the current going into the UTH to keep it at a consistent temp. A rheostat works similarly to a dimmer switch. It will lower the current going into the UTH. The problem with a rheostat is that it's only accurate in rooms that keep constant temps. If your external temps fluctuate, your internal will also, so a rheostat requires a lot more checking and tweaking...