The problem is your existing pump runs on 'high' voltage. What you're asking for is for a converter to take 12v and step it up to 'high' voltage. It ain't gonna happen, and if it does, you'll get 1 'pump' out of the bettery.
You can spend about $500, give or take for a pump system that works off of BOTH AC and DC voltage. The Basement Watchdog is one such system. (Home Depot has them.) I'm looking into it myself but am waiting because the real 'cause' of my water problem (when the power goes out in a heavy rain) is from a neighbor's downspout. It empties 750 sq. ft. of water onto my lot. Since he's seriously considering vinyl siding, they're gonna move the downspout to the OTHER side of the house, at that time. Moving the downspout may actually eliminate my need for a pump at all. (That's my situation.)
For now, you should check that you have proper sloping AWAY from your foundation. And check that your gutters don't leak (during the next rainstorm), or overflow. And see if your downspouts are 'dumping' water near your foundation. (There should be at least a 3' extension AWAY from the foundation if your emptying out onto grass ...) Mitigating these circumstances may lessen the need for your pump to work so much. (Where does the water from your pump run when it comes out the 'other end'?) Check that the water coming out the drain line isn't running back into towards the house/pit.
My best to ya and hope this helps.
PS: I don't want to put 2 pumps in my pit either. That's why I'm leaning towards an AC/DC pump ...