My dads' one of the smartest men in the world..{ -; and is also an electricain.
I was interested in this question myself... just called him..here is his answer..

Yes..if you wire the batteries positive to positive and negative to negative you are safe and that is 12 volts..which you can run your motor longer on it.

IF you wire poistive to negative you will have 24 volts..and could ruin your motor as it is wired to run on 12 volts..

UNLESS you have a 24 volt trolling motor