I have a 2001 carver with gen, 3 batteries one battery is for gen only with it's own switch. The other 2 batteries are for the engines and house usage. I have a battery switch bank 1, bank 2 and both. When switch is on bank 1, or bank 2, it will start both engines and house battery for the boat. Bank 1 or bank 2 the one not being used is isolated so it really on reserve. When batteries are weak you turn the switch to both position battery bank 1 and battery bank 2 are connected in parallel both batteries are being use.This is used when your batteries are weak. It says to switch to either bank 1 or bank 2 when the engines are running. When you are running the engines use bank 1 one day and use bank 2 another day it will keep them charge, of course if you use your gen the charger will be charging both batteries all the time.