although you can wire two of the 6V lights in series, if you have a burned out bulb in one, both will go out. Before you purchase 6V find out which bulb the lights use and if there is a 12V replacement bulb, then just change light bulbs.
Napa carries (may not stock) 12 volts amber sealed beam that fit 5" fog lights. Just replaced one bulb after 25 years so decided to replace both at same time.