Actually Mountain,
As long as you connect positive to positive and negative to negative it will all work as well as it can. At solar trickle charger currents, balancing is just not an issue.
But, no matter what you do, expect that one battery will end up not being as well charged as the other. This is a fact of lead-acid batteries in parallel. Use them as long as you can, and then proceed to do the 6V in series.
Batteries charged in parallel has always been a bad idea. This is just the sort of thing that I did for a lot of performance cruisers (read retired racing keel boats) before the depression.
Matt
