Dimmers work just fine with CHEs. I have about 4 running in my snake room. As long as your house as a whole is thermostatically regulated, and room temps don't deviate, you will be just fine.
Also, two 100w CHEs, combined with two heating pads is going to be MAJOR overkill for a 40 gallon tank. I would honestly just go with an oil heater in the room to raise your ambient temps. DeLonghi makes a Safeheat oil heater that is extremely efficient and wonderful at safely raising room temps.
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