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.