!st, you wont save money by going with 1 larger heater you will use more electricity because you 1 heater in 1 (will have to stay on longer to heat your tank) area as apposed to two 200 watt heaters (will be on for less time heating both sides of the tank) one on each side of the tank and will heat the tank quicker. In any case 200 watt heaters are good for up to 55 gallons so both of your heaters combined really aren't adequate. A 400 watt heater is about the correct size, and using two of them is the right formula, Both of them together will greatly reduce any large temperature swings and if one does fail the other is powerful enough to heat your tank. Redundancy is a great thing. And if your asking why 2 200 watts aren't adequate and 1 400 watt is, is because the 400 watt heater coil is longer and heats more of the water column then the 200 watt heaters which are shorter. JMHO
