Common ground between batteries

I'm redoing my boat which includes a new front deck capable of supporting a Xi-5 Motorguide tm. It is only a 12 volt, but I'm considering hooking the rear battery up in parallel with the front one for extended run time. This would also satisfy the requirement/recommendation to have a common ground (to prevent electrolysis), but I'm unsure of the wire gauge size to use for a run of approximately 16 feet. I presently use a 2 bank MinnKota on board charger and understand that I should break the connection between the two batteries when charging and could do this with a simple disconnect switch. Would someone who has knowledge/experience please weigh in on this. I will not have any issues with my ff as I use a 12v dry cell battery for it. Also, if I did not connect the two, what size wire should I run just to accomplish the common ground? Feel free to speak about any other issues I may not be considering.

6 gage wire should be adequate. but I would consider a battery cut out switch and switch from battery A to Battery B this way you know when you are about to get in trouble with your starting battery also, pay close attention to your fusing and its better to go overboard here than it is to burn something up. I've towed enough broke down boats, (Usually dead batteries) that I keep a set of booster cables in my boat tool box.

Its an age old argument but myself, I would never use my starting battery for a trolling battery. I would instead opt for a second battery for my TM, Hook the 2 bank to the TM batteries and use single bank or equivalent for my starting battery.

Yeah, 6 ga is plenty. I was just wondering if it needs to be that heavy. The disconnect I mentioned would be between the batteries and would do the same thing you are talking about. I do understand what you are talking about with the starting battery being drained by the tm battery. The reason I don't want to put another battery in for the tm is that this is not a large boat and I don't want any more weight ...especially up front. It seems like I may have read that for just a common ground 16 ga is adequate. After all, it would not be carrying any current. Thanks for your input.