FWIW, I store batteries in my basement over the winter. About every two
weeks I place a small "half-way intelligent" charger on each, in turn, and
bring each up to "full". I think your parallel charging is not a good idea
and may have partly destroyed one of the batteries. Diode protection (for
charging in parallel) might work, but it will probably change the voltages
involved and may confuse an intelligent charger. I suggest individual
charging.

Bill
W2WO