So you have no mean to isolate one of the engines or the house loads from the rest of the system? So let say you get a shorted starter or alternator or whatever can happen Your only option is to turn off that single battery switch shutting down the entire boat??
Makes no sense, no builder would set up a boat that way. I have a feeling a prior owner did some creative wiring on that boat
The bare minimum should be one battery connected to one engine with an ON OFF switch. House loads shoudl be connected to one of the batteries with an ON OFF switch or better connected to both battery via a selector switch which would also serves as an emmergency parallel switch.
But to have both batteries going to a single 1 2 B switch powering house and engines is just plain stupid