Lead acid batteries in parallel are hard to manage for charge. A better
solution is a starter battery for the engine and a deep cycle (house)
battery for everything else. Switch between the two for charging. Just
remember that most outboards that size usually put out only 10 amps or
less which is fine for topping off the starter battery but you are not
going to be able to charge the house battery at much of a rate once it
is down. You will have to shed load down to what the OB charging
circuit will support in order to maintain voltage. Best to charge the
house battery to full charge with shore power to before you go out. If
you need more, add another fully charge house battery. I have 3
batteries on my toon boat.

Ron