6v to 12v motorcycle conversion
You must make sure the coils are in phase (their windings are at the same position relative to the magneto magnets) and you must plug the 2 coils in series to add their voltage. Plugging them in parallel will only add their current, with no voltage increase.

Your coils will probably output something greater than 6V, as you can only regulate 6V with a higher voltage, because of P-N junction voltage loss.

I don´t think the 6v to 12v will work in this case, since you are producing "12V" in the AC coils. It would be best to use a voltage regulator adapted from a 12V motorbike.

You will also need to use a step-down converter for your CDI (if your bike has one) because was made for 6V, so it will probably get damaged at 12V.

BUT: i think this is not very smart, as leds work much better at lower voltages. 12V led lamps usually carry power-consuming current limiting resistors.
Try to find 5V leds, they will work with your 6V motorcycle if you add a diode or 2 in series with it´s power supply, to lower the voltage to something around 5V (no precision needed here).

If you can´t find 5V leds, grab your 12V leds and measure it´s current at 12V, then change the resistor so that you get the same current at 5V.