save yourself the hassle of manually isolating your batteries with that switch and look into installing a ACR (automatic charging relay ). Search battery combiners.
Basically they work by sensing voltage from your boats charging system, open a circuit and allow your boats alt. to charge both the starting and house batterys. When you turn off your boat, the ACR disconnects and isolates the house and starting battery. You can also use with a perko switch so you can manually combine both batteries if you need too, or completely turn off all batts.