IMO, if you hook up two 12 volt batteries in parallel, when new and roughly factory equivalent, they will mimic each other. As you charge/discharge them, one battery will tend to do more of the work and take more of the wear over time because of the way they are wired and their inherent internal resistance. As you discharge/recharge them, one battery will charge more than the other because it is working harder and sees more charge. (or less depending on which battery gets the higher charge voltage) That's why, in general, 2 simple 6 volt batteries wired in series (to make 12 volts) are normally used in simple applications. With more than 2 batteries wired in series/parallel, things can get more complicated.
Edit: That said, if you want the cheapest solution, just get a dirt cheap 12 volt automotive battery. If you don't boon dock and are always connected to shore power, IMO it's all you need.
Google: "The 12 volt side of life" if you want a more detailed explanation.
Chum lee
