Heres some batteries for ya 8 volt 750 pounds, $1000.00 each. Just noticed this thread is almost a year old.
yes 2 - 6 volts wired in series will give you 12 volts the problem is you have a 12 volt charging system . You cant charge batteries in series only in parrell. MrC.
If the two 6-volt batteries are the same A/H rating, and in the same condition (not one new and one 10 years old) you can charge them in series.