You can do what your saying....run the TM off two batteries wired in series for 24 volt and hook your out board motor to one of them to crank it on 12 volts but.....you will not be draining your batteries equally. And if you pull crankbaits all day besides running your electronics , live well etc. off of the battery you have tapped for 12 volt.....you may not have enough left to crank your outboard. Best to have nothing hooked to the TM batteries except TM and a separate battery for starting your engine and running the other stuff in the boat.