2 heaters are more for backup purposes. Incase one breaks or goes out. A lot of discus people do this just to protect their stock. Because as we all know, discus cost a little more then your average FW fish. Plus like Fuzz said one can mess up and either fry your fish or freeze your fish. With 2 in the tank it kind of balances that out and keeps it from happening.
So you don't have to run 2 at full power.