Two heat mats are fine, but on a thermostat of course. The only problem with heat mats in a really chilly room is that heat mats don't help with raising the ambient temperature. You might look into using a Deep Heat Projector (DHP) instead of a CHE. They don't wreck humidity the way a CHE does, and they do a really nice job of warming substrate. I put a couple of big pieces of slate rock right under mine, and the slate absorbs and then radiates the warmth. DHPs tend to sell fast, though it looks like Reptile Basics has them available at the moment.

But yes, two heat mats will certainly help.

I'll co-sign the recommendation to use a Herpstat thermostat!