Actually a rheostat should work with both of them, but it's not a very good long term solution. Also, there may be a problem with one producing more heat than the other so you might have a hard time dialing in the ideal gradient.

Either way, a rheostat is okay in a room where the temps are consistent all the time. Of course most room temps will fluctuate, so the temps in the cage will as well. As soon as you get a chance I would get a decent thermostat such as a Johnson, Ranco, Herpstat, or Helix.