One lead from positive side of '6v battery one' goes to the thing you are powering, one lead from '6v battery one' negative too '6v battery two' positive, one lead from '6v battery two' negative to the thing you are powering = 12v
We use this on most of our tugs as this gives even ballasting and the option under a differant wiring loom to power up 6v.
Regards
Pirate