Here is what I know about trickle chargers - on 12v vehicle batteries. Without abuse, my car/truck batteries normally stay good for about 10 years.
I use a high quality trickle charger with desulphation capability (Battery Minder, Batter Tender, NOCO) and add pairs of leads at the initial juncture, then hook up to as many as four 12v batteries. They work - keep all batteries up to charge - but if and when one of the batteries finally gets weak or dies, that can/will affect the function for the others attached. So, check all of them periodically.
I think you should be able to use almost any 12v trickle charger to charge two 6v batteries connected in series.
