I'm assuming you have one of those zoo-med UTH's? Those can develop major hot spots and malfunction if not controlled by a good tstat.

You can control multiple heat sources on one thermostat given 1) the heat sources don't exceed the max wattage of the thermostat 2) the heat sources are identical 3) the cages are identical or very similar in size and placed in the same area.

All in all, I'd say using a thermostat is absolutely necessary with UTHs, flexwatt, CHEs, everything. These heat sources can get extremely hot if left uncontrolled for long periods of time. I've seen more than enough burned snakes to know this.