[TampaFla]

Seen this done on boats (power & sail) so probably you run into the same problem on cars, - shortened life of both batteries unless you add an "isolation" switching circuit. Try "Boatowner's Mechanical and Electrical Manual" (Nigel Calder). It's in most libraries and goes thru matching the batteries (parallel vs. serial) with the rest of your needs and equipment as well as the amps vs. volts issues. However marine batteries are expensive and in awful locations so replacing a set of car batteries frequently may not be an issue.