don't mix them if you can help it at all
the reason being when they are paired in series to make 24volts one will likely be under the other battery in both charge and capacity, which makes getting the two balanced sometimes a real pain.
if you do pair them new and used, be sure and fully recharge the older batteries and do an equalization of them before interconnecting them with the new batteries, probably a good idea to do the same with the new ones too, that way they are all about as good of shape and as close to charge and capacity as you can get them to start out.
a couple years back i had a matched bank of 6volt batteries, all the same age, 4 series'd together to make 24 and the other two series's to make a 12volt bank... long story short while doing an experiment and tapping off midstring i drained 2 of the 4 in the 24volt bank, resulting in an imbalance
it took me a week of charging, equalizing, cycling , rinse/wash/repeat
till i was blue in the face to get them back in line with one another.
without having done so, i found that the better pair would reach full charge and then climb up which caused the overall voltage to hit regulation limit before the pair of batteries that had been worked down were not even close to half charged. over time the result would be damage due to overcharge of the better pair and damage due to undercharge on the other pair that had been overworked in the testing.
moral of the story? don't mix new and old, or rather batteries of differing charge and capacity (actual, not advertized) unless you really go to all the effort to get them balanced and keep a really good eye on them. which i am sure you were planning to do with your charging regime, your hydrometer, your logbook and periodic equalization... right?
bob g
