I have two heaters (Ebo Jagers) and most of the time only one is doing the work. The accuracy of heater is +-n degrees. It takes a little time to iron that out. If the adjustment is + or - 2 degrees and both heaters are set at 78 it is possible for one to click on at 76 degrees and the other not come on until the temp reaches 79 which will never happen.
I still prefer two heaters because it reduces the odds of my fish being cooked because a more powerful one gets stuck in the on position.
I have also heard about uneven water temperature issues in a large tank, but, unless the aquarium was ABSOLUTELY HUGE, I find it hard to believe due the the amount of water being moved by filters or airstones in the majority of aquariums.