Until you get the second t-stat you can pick up a lamp dimmer at a big box hardware store. They cost about $15 and work as a short-term solution, and are ok as long as the room ambient temp doesn't swing. They work by limiting the power going to the heat pad, but they don't adjust the power so the pad stays at a constant temp like a thermostat does.
ETA: just read that you had the CHE on the dimmer. I wouldn't use a CHE with a plastic tub either. Put the dimmer on the second UTH and dial it back so the belly heat is 80-81*F on that spot. Put three hides in the tub, one over each UTH and the third on an unheated area, and your critter can figure out the best place to be.