I run 2 heaters on my tank in the event that one fails, there is always the backup and plus, with 2, you are assured neither is working too hard. Both are high enough to sustain the tank if only one were to work.
Ditch that heater before it boils everything. Heaters sometimes have a tendancy to get stuck in the "on" position and then there really is no way to fix them. You just have to get another heater. BTW that brand of heater is not the most reliable. The stealth heaters should work out for you a lot better.