I know I've seen one. I'll try to bring up a picture.
Yes, Ralph Davis ran into this but it was dead in the egg. http://www.ralphdavisreptiles.com/im.../twohead_2.jpg
The San Diego Zoo had a two headed corn snake that produced 15 normal(not two headed or anything) offspring. So there have been other two headed snakes that have been able to live healthy lives.