Good point, Malkore, but perhaps not for the reason you intended. Two undersized heaters will not boil your fish, if one fails in the on position, the way an oversized heater will. I was just reading on another forum, where a brand new (calibrated) Jager heater cooked a tank of rare and expensive corys (around $200.00 worth). I have all, cheap, hang on back heaters that have, occasionally, stuck on, but none have been of sufficient wattage to overheat a tank. I have one Longlife 25w, that is over 40 years old, and still working. Regardless, the point is a good one, and good advice when planning a setup. The initial cost is more, but the long term peace of mond is worth it.
Join a fish club. Don't have one in your area? Start one.