Classic/Antique Car Repair: Low beams wont work - high beams will, filament bulbs, double filament

Question
My son has a 1978 Mercury Marquis, the high beams work and the low beams will come on with the high beams, but will not work on their own. Any suggestions?

Answer
The headlights in question, the outer two, are double filament bulbs and there is a good chance that just the bulbs are blown. And yes they can blow within a very short time of each other as they have burned the same length of time. If the bulb does not correct the problem get back to me please.
Brad