I understand what you are trying to accomplish.
We have used rheostats with heat tape any number of times over the years. They just don't give the same level of precision that a high quality thermostat does. Rheostats can be used, but they are def not ideal.
I wouldn't recommend the joint rheostat/thermostat setup that you are suggesting. I have seen it recommended before, but I always thought it was an odd setup, and I can't imagine it working all that well.
I think a better choice would be two high quality thermostats. One at the proper set point. And the second set 5-10 higher as a fail safe.
We used this setup for years with our incubators, with great success. Jeff Ronne The Boaphile used to (still does?) sell a double Ranco thermostat prewired precisely this way. Those are the ones we used for our incubators.
You could accomplish the same thing with the Vivarium Electronics thermos, and they are especially nice because you can stack them.
I would rather see you go in this direction, even if the intial investment is a bit higher. You will get more precise output, and a safer long term result. Best of luck.
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