My tubs are in an enclosed rack system made by Reptile Basics and I have no issues with low temps or humidity. I have 1 thermostat running 16 tubs in four racks.
I also have cages made by Reptile Basics and have no issues with temps or humidity. I have one thermostat running each three cages.

I would suggest, before buying another t'stat, that you maybe check out Reptile Basics offerings; it could be good for your situation