12V or 24V fog lights?
If they are 24V on your 24V system, the wiring would be the same as it would on a 12V system (the 2 batteries in series acting the way as the single unit in a 12V system).
If they are 12V fogs, you should NOT create an unequal load on the batteries. It sounds like you might have it set up that way currently.
You will want to step the voltage down somehow prior to the lights. This can be done with a properly sized resistor in series with your lights, though you'll want to watch your current requirements.
You could use a voltage converter to change your 24V to 12V.
As they are fog lights, not your headlights, you could connect them in series, but then, if one fails, you lose both.