If I connected 2 6v batteries in series I could use both batteries (12) volts for starting.  A 12v alternator to charge the batteries.  NOW you tap off in the middle of this battery "system" for 6 volts to handle the 55 bird.  The dilemma comes in with "positive" ground for the bird.  You can change it to 6v neg. gnd.    12v starter would be neg. gnd.  A light bulb will glow with current in either direction.  The only accessory that needs correct polarity is the radio. Just a thought.  This is known as "tuning for maximum smoke".   Probably just cheaper to go all 12v.