Correct River. One baby forms, the other conjoined twin is attached (like a parasite) in some form to the developed baby.
This story explains it better
MIAMI (Reuters) - An international team of doctors hopes to operate in the Dominican Republic next month to remove an undeveloped second head from a baby girl born with one of the world's rarest birth defects, caused when a conjoined twin fails to develop in the womb.
The baby, Rebeca Martinez, was born in mid-December at a hospital in Santo Domingo with the head of an undeveloped twin attached to the top of her skull, facing upward.
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Her condition, craniopagus parasiticus, is so rare that there have only been eight documented cases in the world, and no known cases where surgery has been attempted to correct it, Hazim said in a telephone interview.
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Rarer "parasitic" twins occur when one conjoined twin stops developing in the womb, leaving a smaller, incompletely formed twin that is dependent on the other. They can form as an extra limb, torso or head, or as a complete second body, lacking vital organs.
In Rebeca's case, there is a gap in her skull where the heads are joined, and the blood vessels are intertwined, Hazim said. The vestigial head is enlarged and fringed with dark hair like Rebeca's but has a poorly developed brain and only rudimentary facial features, he said.
http://www.reuters.com/newsArticle.jhtml?type=topNews&storyID=4196661
What is so sad is that the unformed twin's mouth is moving when the other is being fed, and just the whole awfulness of one baby having no real chance ever.