Buzz, back in the si.......... er, when I was a kid, and cars were just going 12v. We would run a second six volt in series and then run twelve volt headlights.
We would also run 12v starters that way, keeping the rest of the car 6
You tap off the 6v for your ignition, and the second six is real small.
Don’t remember exactly but, your two sixes are charged up by wiring them up parallel with a couple of diodes.
Man I bet the bike drew a lot of attention.

Got any photos?