Extra heaters can't hurt. You should look into a controller. I just bought 6 of them after cooking a Cuban gar with a failed heater. it adds an extra layer of safety. I set the controller to 80 ish and the heater to 85. If the heater breaks in the on position it will never get above the 80 set point of the controller. If the controller breaks in the on position the heaters are set to 85. Hopefully both don't break at the same time lol.
Also it's easy to put the heaters in the sump and the controllers temp probe in the aquarium. Heaters in the sump stay on until the temps in the aquarium reach set point. No ugly heaters in the aquarium.