There are PLENTY of people who use two UTHs on one tank. They don't take up the whole bottom, but close to it.

I would suggest doing this with a thermostat though so the temps are more constant. But since you don't have a thermostat yet, I guess rheostats are better then nothing!

The instructions on the UTH says a lot of things. Like not to use it on plastic, but obviously if you use a thermostat you can do it no problem.

Think about it. The temperatures that we set are heating elements are never much higher then the temperature that the air can be outside. 95*+ summer days are not unheard of and you don't see all the shop windows cracking or all the plastic bins melting!