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Re: 2QM20 alternator wiring question
You shouldn't need to add the second switch, since there should (!) be a starter switch already, usually a key switch that powers the starter solenoid or starter relay and the starter itself at the same time.
In production boats, you can save a yard of heavy wire on every boat, by running the alternator output to the starter and then daisy-chaining it to the positive bus, instead of running to the bus and then back again to the starter. It really is that simple: this is done to save a yard of wire and a couple of extra minutes of assembly time.
You should be able to just pull the wires off the starter + terminal, and run a new one from that to the new starter battery terminal. Leaving the key switch on the old house bank, or moving that circuit as well. (If the main battery switch is OFF, the key won't start the engine unless you've also moved it to the starter battery.)
In any case, make sure to FUSE the new battery-to-starter wiring, usually just by putting a heavy primary fuse on that starter battery. The old standard was to use "fusible link" wiring, but that's considered a fire hazard now, even when it is run in insulated covers.