Before you bought it, was your boat previously used for charter work ?
The reason I ask this is that when I bought my B36, it was a charter boat and I was required to fit a switch and wiring which would allow the domestic batteries to be used in an emergency to provide current to drive the engine starter motor. This crossover if left switched on would allow your domestic circuits to be powered from the engine battery. To try to ensure that charterers would not use the change over switch inadvertently, I placed the crossover switch in an area well away from the main battery switch, and the key needed to operate the switch was secured inside the battery compartment using two screws. That way a conscious decision was needed firstly to select a screwdriver, unscrew the securings, and then find the crossover switch. As a result the switch was never used.
That might explain why you have two battery switches, and what the second one is for. Tracing where the heavy cabling goes, and looking to see how the cables connect to your domestic batteries might give a further clue. Good luck.