Using more than one heater is recommended since heaters will fail. It is not a question of if, but when. Having a second heater will keep your tank temperature from fluctuating wildly if one heater fails. Heaters will failure by either not turning off or failing to turn on. Both are problematic, but usually a heater not turning off is the bigger problem, since it can result in the tank overheating. If you have two 100W heaters, and one does not turn off, then the other should not turn on and your tank should not overheat. On the other hand, if you have only one 200W heater and it doesn't turn off, that could spell disaster.