To the original question. I think "eric" is right but here is a suggestion. Take a pad and pencil and sketch what you can see. I think you will find three red cables off of the back of the Battery Switch.
One should go to the starter or thereabouts. That post should also have a wire to the DC bus behind the panel. This is the 12V for your boat''s electronics, pumps, and lights.
Another red cable should go from the back of the switch to your starter battery. The third red cable to one of the 6V house batteries. So in switch pos. #1 there is only 12V from the starter battery to the starter(and everything else). In pos. #2 there is 12V from the house batteries to the starter and everything else.
Now draw the wiring from the alternator. If you find two wires can you trace them? On a single-wire alternator the one wire goes to the starter since the wire to the battery switch is already there. But with a 2-wire alternator you probably have a wire directly to each battery bank. If so then the battery switch position does not matter, if the engine is running then both banks are being charged.