I converted from a single 12v to dual 6v this past summer. Points made about doing the math for battery capacity are valid - you have to add the amp hours from your 2 current batteries together, and then see what you can find in a 6v that has the same or greater amp hour rating. Yes, when in parallel, you add the amps of the 2 batteries together (the 12 volts), but when in series, it is the amp rating of a single battery (the 6 volt).
BUT - as noted, most 6v batteries are capable of deeper discharge without harm than most 12v. So you get more usable amps out of them. That's the big plus and the main reason for switching.
There is a size difference, so you have to check dimensions and make sure everything will fit. Unfortunately, every brand is a little different. Not huge - but if your compartment is tight, you might have to shop around....
