Do you have an idea of what brand pumps you plan to get? I would strongly advise against getting the Countryline ones sold from Tractor Supply Co. Those are utter rubbish from my experience.
I like the iON company, particularly the Storm Pro series. This would be the main pump:
http://www.sumppumpsdirect.com/iON-Products-BA-75M-Sump-Pump/p2879.html. It is meant for a harsh environment with high flows and small solids, if necessary.
The backup looks just like it, but 1/2HP version.
They are designed to pump as much water as most high-end submersibles, yet use half the starting and running amps than most. The main one would run on just under 8 amps at around 4,500GPH. A comparable pump in other popular brands can be around 20 running amps for the same amount of flow. The marine battery system that will serve as a backup will power the main and backup pump together, if needed. Otherwise, with the lower amps, one marine battery of high capacity could power that main pump every few minutes for nearly an entire day. Batteries can be added and chained if needed. Automatic system that recharges by itself and switches over when there is a loss of power. Rare for us, but in central Georgia we have had tropical storms go over us and flood lots of areas. Or, when it rains a lot, trees can down lines and it might take hours to get power restored. That's another reason why I like those particular pumps. Very good battery backup life, yet with main pumps and not those silly 12v cheap backup pumps. Plus, they keep the regular power bill down even if pumping for days. Good reviews from everyone that bought then. The float switch can be changed to an electrode version without a float in order to get around the half of a foot or so differential and have it pump an entire basin out at once, instead of constantly coming on pumping out every half of a foot of water.
I would order from Sump Pumps Direct. They are a division of Power Equipment Direct. They have amazing service and actually know their products. I have dealt with them before.
I mused about the pumps at home improvement stores, checking then just out of curiosity. They all carry the cheapest, smaller pumps like Flotec, which has a horrible reputation for breaking. Usually they have nothing over 1/2HP and sometimes not above 1/3 HP.