Nice Equipment GLS!
Randy2625- You can wire two battery up in two different way.
1st is called a parallel connection which is what GLS did like already said you simply add another battery and connect the positive to positive and negitive to negitive terminals. Then pull your negitive charge and positive charges form seperate batteries.
2nd is call a series connection which is where you connect a positve terminal from one battery and a negitive terminal from the other then pull you positve and negitve charges from the two remaining terminals again on seperate batteries.
Which one you will need is depenant on what the use is. The series connection is what most disieltruck are setup on. They require alot of voltage all at one time. A parallel