Contrary to what was presented at Goshen, depending on how you wire the batteries you either get double the voltage or double the amp hours, but not both (assuming 2 batteries).
Batteries, using round numbers
6 volts 100AH times 2 batteries wired in series = 12v at 100AH
12 volts 100AH times 2 batteries wired in parallel = 12v at 200AH
Now if you have the room and want to drop the coins:
Four - 6 volt batteries
6 volts 100AH times 2 batteries wired in series = 12v at 100AH - bank 1
+
6 volts 100AH times 2 batteries wired in series = 12v at 100AH - bank 2
The above two combos then wired as:
Bank 1 - (12v at 100AH) wired in parallel with bank 2 (12v at 100AH) = 12 volts at 200AH
Although I don't currently use one, the
Bogart Trimetric is very highly rated by all who do use it to monitor battery status, both in and out.