If you put two batteries, 2 6 volt in series, or two 12 volt in parallel make sure both are close as possible in build dates, (ideal would be to have two batteries with sequential serial numbers i.e. ...1 & ...2. If they are not perfectly matched the weak one will bring the other down to its level. Also, do you do a lot of dry camping? If you only go from plugged in camp site to another plugged in camp site IMHO you do not need to hassle with two batteries. One gel cell will be long lasting hassle free alternative. A little pricey, but one gel would be cheaper than two lead acid!