You have 3 batteries, right? One marine cranking battery and two marine deep cycle batteries. Blue Sea Systems makes a 4-way battery bank switch. Make-before-break contact design allows switching between battery banks without power interruption. There is a Off position, battery A position, battery B position, and a BOTH position. I would add this 4-way switch for your 2 marine deep cycle batteries. And leave your cranking battery as is.