Assuming it is 12 volt positive ground
If using 2 12 volt batteries in parallel, connect both negative posts together and to starter solenoid or switch. Connect both positive posts together and to frame ground
If using 2 6 volt batteries in series connect negative of one battery to starter, the positive of that battery to negative of second battery, and positive of second battery to ground.
For safety reasons, Make your connections to the starter first. Ground connection to the frame should always be last one connected, first one disconnected