That's the same system as used in a lot of dirt bikes. One bar is only for the ignition (its the magneto bar).
The other bar is the single phase alternator. You need to disconnect one side from ground, then the two end wires (disconnect the one to the headlight) go to the bridge rectifier. After that it will probably make enough voltage to charge a 12v battery, many people make that mod. You won't get a lot of current, maybe 4-5A max, but it's enough to run a 30W (3A) headlight and charge a 12v battery and run turnsignals and brake light etc.
You can also buy 12v LED tail/brake lights and LED turn signals too, which reduce current required.