Al your much better having two batts than one high cap batt. With one batt you take the risk even at low volume levels of draining the one batt to a level where the boat won't start, not good if you suddenly have to move! When your running multiple amps you really should separate them from the starting circuit by using a batt switch or something like i use, the Blue Seas ACR which keeps the batts charged without messing with them.

If you need additional info on how to do it and what you'll need let me know and i'll be more than happy to help.