A couple of Ford-type starter solenoids can be used to fabricate a series/parallel system where both batteries run and charge on 6V and are only run in series during starting. That system has been used as a 12/24 volt setup on big Diesel trucks in cold climates for over 60 years that I can remember, and probably long before that.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!