Look up the thread about multiple heaters in tanks. If the heaters have temperature settings you can set them up in series so they kick in based on how much colder the water gets.
Also if you use backgrounds on the tanks you should consider using some kind of insulation behind them as well. Walmart generally sells the half inch camping pads everywhere in the US for about seven bucks. Just let it relax a big, cut to fit and stick it between the filter and the tank.
Power filters are also a heat-loss source in cold air, a couple layers of masking or silver tape will work wonders. You can put a cut piece of cloth to put over top of them to reduce air circulation evaporation without making a hit in air exchange. If you're inclined you can build a layer of the same thick foam onto them everywhere but the motor assembly.
I almost always advise some kind of forced aeration in tanks but in very cold climates its much more efficient to almost seal the tank with plastic wrap and use forced aeration. The pump actually heats the air a little and a single air stone is just as effective as an open topped tank.
In winter down here in Florida I don't run the furnace until the outside hits 38°F and I get along with closing, insulating and the second heater arrangement.