What he is doing is just using the two batteries in parrelell to make the trolling motor last longer through the day. You can hook up 100 batteries together and still only have twelve volts as long as all the positives are hooked together and all the negatives are hooked together. He will get more running time by the way he is hooking two batteries together. To get 24 volts from two batteries you need to hook the positive from one battery to the negative of the other battery and then the outside positive and neg will give you 24 volts and that is called wired in series. I hope this makes since