I agree with MUNI.
The stealth itself has recommended wattages based on tank size. I have a 55gal and I thought the 250 watt was recommended. You can look it up. When I first started my tank I used 2 heaters as well. After a few years I removed one mainly because the heaters never seemed to wear out. If anything I've had the opposite problem. I've had one of the heater get stuck and not shut off. So now I just use one.
The tough part with using two stealths is that they don't have a running light. You could set both heaters to the same numeric value but you can't really tell if they are both sync'ed.
I highly recommend a good thermometer and keep a second heater around in case the first one does go.