If you want to use 12 v on the starter and 6 v on the coil, here is how to do it. Disconnect the other wires from the starter except the battery wire. Disconnect the battery or power wire to the coil. Hook up your 6 v battery or supply, -ve to ground and +v to the coil. Hook up the 12 volt battery to the starter feed. I am presuming here that you have a foot starter. When you crank the motor, the coil will be running on a full 6 v. Of course when it starts, it won't be charging because the electrical system is disconnected.
Oh, make sure that the disconnected wires are isolated and not touching ground.
