If your battery is in good condition you shouldn't have any problems. I've run 2 units (wired like yours) and a 360 for that time frame and didn't have starting or unit issues.
-Do you need to have both units running all the time?
-Carry jumper cables on the boat, if you need to, jump to a trolling motor battery.
-Turning down the brightness of the screen, and turning off the transducer that's not being used will save energy too.