Well, I'll put in my two cents, but I'm not an expert here to be completely honest. In basic electronics theory, one can step-up the voltage from 12 volts to 24 volts. In doing this, your available current will be cut in half. That may or may not be enough power to supply to the lights (I don't know what their power requirements are in AMPS). I also don't know where you will find a step-up transformer. The flip side, which you said you don't want to do, is to wire two 12 volt batteries in series, but then you not only have to buy 2 more batteries, you need a way to charge them. Post your results, I'll be curious.
Transformers only work on AC systems.. On DC you will gt a spike when the circuit is made and broke, otherwise it flatlines to, nothing
I have seen 12 volt DC LED's for about the same price..
When used with a step up converter the 24V lights work just fine on a 12V setup incase anyone's wondering. Check eBay or amazon, they're around $25. I've sold the cheapo $39 lights but have a third light which I bought thinking one of my lights broken.
Turned out wasn't grounded properly, and all three work fine. I gotten several inquiries about the lights. The remaining light is $125, which is how much I paid for a single. They can be found individually on eBay for around the same amount.
For what its worth, the newer ones are already 12V and perform better. These look nicer though IMO.
A wall shot of low beam is posted below. For reference, I've included a stock photo.