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.