I first read about Edward Mordake in Re/Search Magazine, which quoted the original story from Anomalies and Curiosities of Medicine by George M. Gould and Walter L. Pyle, online here about halfway down the page. At the time, I assumed the story to be apocryphal, but over the years I've realized that it might be true, except for the voice (obviously), which no one else could hear. There are modern examples, as in this video of a Chinese man:
Our developing operatic version may be real or not real. It doesn't really matter. In Douglas Kearney's libretto, the other face is presented as an example of chimerism, a son that devours his sister in the womb, which is biologically unlikely. More probable is that Mordake was a variant of a conjoined twin, like the slave owners, stars and truly 'Siamese' twins Chang and Eng Bunker, whose commingled liver is on display at the Mütter Museum, a prime tourist spot for all those intrigued by the fringes of permutation of us, we featherless bipeds with broad, flat nails.