Wallyworld sells a switch that goes on the negative side of the
battery. It has a green round knob on top and sells for around $5USD.
I would think that you could connect the 2 Positive terminals together then run a ground strap from the frame to the negative side of the switch, on each
battery, then you could turn off either
battery as you please.
You would not connect the negative terminals together.
That would be 2 ground straps. Each battery would have one of those green disconnect switches installed on the negative terminal and the ground strap to the frame.