Short of the heater being stuck on or off, I have never noticed a difference in using one heater or two as long as water is moving. If your aquarium is like a stagnant pond, then there will obviously be a hot spot, but if water is moving around, (filter moves enough water), there shouldn't be a problem. I generally use one because I don't like the look of two heaters. Like I said it doesen't really matter as long as the heater doesen't break and you have decent water circulation. All IMO of course.
you can only run one heater and be fine. on the other hand i have at lest 2 heaters in almost all of my tanks. i also keep backup heaters just in case also. as rbishop said heaters can fail.
oh i use visi therm heaters. been using them for years and never had a problem yet.
I have been using the stealth heaters and I have only had one go bad on me it stays on constant now so I replaced that with a visi therm heater. I only run one heater in all my tanks and I place it right next to my intake on the tanks that don't have a sump but the thanks that do I keep it in there and all my water is the same across the tank. I do have extra heaters not being used just incase one goes bad.
I've been using a single stealth in mine over the last 3 weeks. I purchased 2 thermometers and have them on each end of the tank. They read the same 78*F. However, as soon as funds allow, I will be buying a second one as a backup and for my QT.