a buddy told me never to put my volt selector switch to 12 volt as this will wear down one battery real quick, is this true or does the system[motorguide]  draw from both baterries regardless of what volt setting you have on ?

I seem to remember that when you are on a 12 v TM setting that the motor pulls from one battery only. My boat came with a 12/24 TM but after a short time I just never ran it on 12 v. What's the point? Run it on 24v but at a lower setting or not as often.