Can you run 2 batteries for a back-up sump pump to double the time it would pump for?

I have a small back-up pump for my sump crock. It is rated for 1000 gmp at 10 feet. When a storm comes and knocks out the power, the little back-up pump runs continuous and within 5 hours, runs the battery down. Can I put 2 batteries in series to double the duration the pump will run? The battery is a deep cycle marine type battery that came with the system. Is there a better battery I should look at getting?