Are you really looking to add a house battery to power accessories or a redundant battery for starting?
For a boat this size a house battery is typically not used.
For adding a second battery all the wires on the Common terminal now go to the common terminal on the new switch. All the wires on the other terminal of the kill switch will go to terminal one of the new switch. The positive of the new second battery will go to terminal two of the new switch.
1997 2400 SCR
5.7 Vortec / Bravo 2