I run double heaters both on T-stats. It is possible to do it with rheostats but they really work best in stable room temps and if you have stable room temps there is likely another way. personally using two heaters is very efficient cost wise but a with the entire bottom of the enclosure heated means a failure will likely kill the occupancies. I would highly suggest a good t-stat for both with a single fail safe. I run low wattage on the cool end so in the event of failure it simply does not get that hot. (17 inch flexwatt is great for this as it gets to about 85-88 maxed out)

I saved 50$ a month on my power bill doing things this way.