I have seen this in cruising boats all the time. I know of no reason not to do it.
Some will say that paralleling battery is not a good thing but the same can be said about serial connected battery, they would only be as good as the weakest battery.
You are getting a lot of advice to go with 6 volt golf battery if you look at the chart.
http://www.trojanbattery.com/pdf/TRJ...dSpecGuide.pdf
you will see you get the same capacity as 12 volts battery. Based on weight. It hard to find the same physical size to compare.
Now JRTJH makes a good point that you may get more battery finding a 6 volt battery that is bigger/better physical size that may fit your space better that would increase capacity over a 12 volt battery that is smaller in physical size. There are many other reasons to go with 6 volt golf battery such as availability and they are true deep cycle battery.
I like cheap Wally 12 volts battery and just plan on replacing every couple year that way I always have new batteries.