Ooooh. I was thinking that if I "reset" the thermostat to 70F (so the middle line on the dial represents 70F)* I could then slowly increase it until they are both 90F. But since one is 8W and the other is (I believe) 16W, I can never have them both the same? I have a dimmer lying around. Can I incorporate it in some way?

*When the thermostat was registering its "set" temperature, I think it got set to the pad's highest; ~115F, because that's the temperature if I leave the dial set on the middle line.