Thermostats can only control the same type of UTH at one time.. if you put a smaller (lower wattage) heater on there too, that one would get the same amount of power as the large heater, and thus get hotter (too hot most likely) Always, always put the same kind of heater on the same thermostat.. they are not made to control more than one kind at the same time.

So.. yes, get a second thermostat. And you don't have to spend more than about $70 to get a quality reliable thermostat (either a Johnson or a Ranco)