The battery for watchdog is supposed to last like 7.5 hours continuous use. Under normal conditions sump pumps don't run continuously.
After battery dies generator can be used to power the main pump and recharge the battery. Guess you could yank car, boat, or lawn mower battery if really desperate.
BTW I think you can connect two batteries up.
I have yet to see my backup system operate in a power outage but under the worst rains so far the main pump turns on about every 15 minutes for about 20 seconds. So maybe and I am just guessing I could go days on battery power only. Granted this is not what the OP was dealing with.
Your only other option would be water powered sump pump. This would work nicely if it can keep decent water pressure and be able to keep up with flow.