On Guenevere, when we were prepping to go cruising I did NOT use an A/B switch.

I felt it was better to use 3 separate single throw, switches.
One is for my house bank.
One is for my start battery to engine.
And one is a combiner, if needed.

I have the house bank switch in the main cabin, and the engine and combiner switch in the aft cabin.

It's worked VERY good for us for quite some time.

Greg