I have a Catamaran with two seperate 12V starting batteries - one for each engine - along with a house bank consisting of 6 6V Trojan deep cycle batteries. I also have 2 alternators - one on each engine.
Is it best to have both the starting batteries allocated to the "1" position then the house bank designated to the "2" position? Or, should I find a way to isolate the two starting batteries from each other? Is there a switch that offers 1, 2, 3, ALL and OFF positions? And if so, would that be the most desirable for my setup?
If you bring both batteries to a central switch, that would defeat the purpose of having one starting battery per engine since you still need the long battery cable runs. The answer is also dependant on whether or not you are always running both engines at the same time. Still what has been said before applies to your case:
1) I would have both alternators charging the house bank
2) I would have either a single starting battery bank or one dedicated per engine.
3) I would charge the starting battery(ies) from the house bank with a DuoCharge or equivalent.