As long as they are on Thermostats or Dimmers, I don't see a problem with it. I actually have 3 on my 20L tank. One on each side to create a warm (92 degree) and cool (82 degree) gradient temps hooked to a Herpstat II thermostat. I also have one attached to the side of the tank to help with ambient temps which is also hooked up to the Herpstat (82 degree) thermostat.
