some of these are different opinions but OHM's law is the determining factor of all electrical theory and discussion ( V=I x R). this discussion reminds me of the old JD split load 24v system-- fully insulated starter, generator with two 12v outputs charging two 12v batteries separately and the lights & other accessories split between each battery. what a PITA and a lot of connections to check and maintain. the best solution was to change to a 12v starter and 12v alternator and basically rewire part of the tractor. also ditch the skinny 12v batteries, make a new battery box , and use two 6v in series or two 12v in parallel