Having the two batteries in parallel would be the most convenient for use, but to do so they should be batteries which are the same, otherwise they will be trying to equalize each other out. If you can find the same type of battery that you already have, I would wire them in parallel. That is what I have, 2 12vs in parallel. If you cannot get a matching battery, then it might be better to keep them separate and use one first and then the other.
Glenn & Beth (Dad & Mom)
David & Audra (12 year old twins)
2006 Dodge Power Wagon (Adventure & Tow Vehicle)
2006 Rockwood 8281SS (Home away from Home)