Deceler8 has the right idea. Wiring the headlights in series provides for a 6V drop across each, which is what is required. Another option is to measure current through the headlight with 6V applied. Let's say you get 3 amps. Then you will need to find a dropping resistor that will take a voltage drop of 6V with 3 amps flowing through it. That value will be 2 ohms using ohm's law (voltage = amps times resistance). Then you will need to ensure the resistor has the proper wattage. Power = voltage times current, or 6V times 3A or 18 watts (minimim). This is an oddball resistor and not something you are going to find at Radio Shack. Going in series is the way to go.
