Lets look at it this way... I will use the TROJAN batteries as they have a great specs guide with all the info available on it.
T105 6VDC GP24 GC2 BATTERY weighs 62 lbs
447 minutes (7.45HRS) capacity producing 6VDC @ 25AMPS
225 AMP HOURS @ the 20HZ rate
Requires two in series to produce 12VDC
When two are connected in SERIES the specs are
447 minutes (7.45HRS) capacity producing 12VDC @ 25AMPS
225 AMP HOURS @ the 20HZ rate
Cost around $80 or $160 for two
27TMH GP27 12VDC BATTERY weighs 61LBS
200 minutes (3.3HRS) capacity producing 12VDC @ 25AMPS
115 AMP HOURS @ the 20HZ rate
When two are connected in PARALLEL the specs are
400 minutes (6.6HRS) capacity producing 12VDC @ 25AMPS
230 AMP HOURS @ the 20HZ rate
Cost around $180 or $360 for two
So in this case you will get more capacity and longer operation in hours from the two 6VDC batteries in series then two 12VDC batteries in parallel for half the cost.
Roy Ken