I first read this story in the New York Times, but I am glad that I ran across this BBC News article, because it has pictures.
I initialized imagined that the two heads both sprung from one neck, like Rachel in A Canticle for Leibowitz. Alas, the picture shows two heads on top of each other.
Her second head had a partially-developed brain, ears, eyes and lips, and if it had continued to grow it would have prevented Rebeca's brain from developing.
The features on the second head reportedly moved when Rebeca was being fed.
Dr Lazareff, the director of Paediatric Neurosurgery at UCLA's Mattel Children's Hospital, said the head on top had been growing faster than the lower one.