Two 6 volts in series will charge the same as one 12 volt and should be charged this way so they are equally charged. Make sure you keep the water level up to normal level, if your old charger normally goes to zero at full charge then it should do the same with the 6's, but it would be best to disconnect the charger, let the batteries set for a couple hours then test with a meter to determine full charge.
Dale & Terri, Lulu & Tiki (our Chihuahua's), New rescue puppy Prince - Pom/Pug mix.
2013 Tundra, Double Cab, 5.7, TRD Off-Road, 4X4, Full Tow Package
2013 Wildwood T26TBSS - Sold
2000 Gulf Stream Sun Voyager M-8357 MH, Ford Trident V10 Gasser, 35 foot.