The polarity reversing switch was an experiment in prolonging ignition point life that didn't work out too well. Most of them got bypassed in the 1950's.
What you're going to need to do a dual 6V setup is a "series/parallel switch", and finding one that operates on 6 volts will be a challenge. It would be possible to do a work-around with a Bosch-style 6 volt relay for battery charging, and a Ford-style 6 volt starter switch, or maybe two. I would need to spend a while sketching out a circuit diagram that would work, but I'm pretty sure it's doable. There used to be 6/12 volt batteries set up that way for cranking balky 6V engines, back in the 1950's and 60's. I used to manufacture them when I was on an assembly line at Hester Battery Co. in Nashville.
Jerry
