That is not the right way to wire your batteries. This was discussed recently in another thread and "the twelve volt side of life" was referenced as a good source of information. Look it up and you will see that the positive should come off of one battery, and the negative off of the other battery.
If you do it as in the above image, the battery on the left will experience more resistance due to the wire connecting them. If you run positive from one battery and negative from the other, you even out the loads.
The 12volt Side of Life (Part 1)