If the 1/2/both/off switch is wired onto batts positives, then you simply connect the solar regualtor out onto any of the batteries. In the both position you will be charging
both, in any other position you will be charging
only the bat that has the sol reg connected.
This is likely the simplest but not always the simplest is the best.
I our boat (one bank for starter, separate house bank):
- NO 1/2/both/off switch,
- relay from engine
batt to house batt (charges house when the alt is running),
- sol reg neg wired to batt neg, pos wired with a (house/start) selector,
So we manually select which bank is being solar charged and both get charged from the engine
as soon as the alt kicks in.
With no 1/2/both/off switch, we never fry the alt. If we were to jump start the engine, I keep a neatly coiled and tied down jump cable next to the batteries.
Your options are endless. See what you have, think what you want, adjust your set up. Done.
PS If your sol reg has two outs, you do not need a selector switch either.