I cannot choose one of the answers given. My answer is I think it is unconscionable.
Vladimir Petrovich Demikhov, Soviet scientist, pioneer in organ transplant, conducted dog head transplants during the 1950s, resulting in two-headed dogs. [YouTube video]
In 1970, after a long series of preliminary experiments, Robert J. White, an American neurosurgeon, performed a transplant of one monkey head onto the body of another monkey. Because the surgery included severing the spine at the neck, the subjects were paralyzed from the neck down. After the surgery, because the cranial nerves within the brain were still intact and nourished by the circulatory system from the new body, the monkey could still hear, smell, taste, eat and follow objects with its eyes.
Ultimately, immunorejection caused the monkey to die after nine days. Dr. Jerry Silver, an expert in regrowing severed nerves, called White's experiments on monkeys, "fairly barbaric." [source]
I'm having a hard time understanding how making a two-headed dog is beneficial for humans. Is there a perceived need for two-headed people and the dogs are some sort of practice run? Just because scientists can do something doesn't mean they should.