I think what you're talking about is a "piggy back" heart transplant:
Two hearts are better than one for toddler
Medical Procedure News
Published: Monday, 25-Oct-2004
Camila Gonzalez now has two hearts beating separate rhythms inside her tiny chest. At 22 months of age, she became the youngest child in the United States to receive a donor's heart while also retaining her original one.
Bruce Reitz, MD, professor and chair of cardiothoracic surgery at the Stanford University School of Medicine, connected a second heart to Camila's on Sept. 16 at Lucile Packard Children's Hospital at Stanford.
The procedure - called a heterotopic or "piggyback" heart transplant - was first developed in England 30 years ago to address a specific kind of heart problem. But Camila is the first child to undergo such a procedure in California and only the ninth child to receive a second heart in the United States. In adults the procedure is rare as well. Out of the 1,200 adult heart transplants performed at Stanford Hospital, only one other involved the piggyback procedure.
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Camila suffers from cardiomyopathy, a heart muscle disease that had caused the blood pressure in her lungs to build to five times normal levels. A conventional heart transplant wasn't an option because the new heart would have failed under such great pressure.
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Double hearts are a good choice for patients whose heart problems cause extremely high blood pressure in the pulmonary artery - the blood vessel that takes blood from the heart to the lungs. Such pressure often builds up in people with a weakened heart muscle, a condition known as cardiomyopathy.
More:
http://www.news-medical.net/?id=5814----
Posted on Fri, Oct. 22, 2004
Stanford doctors give ailing toddler a second heart
Associated Press
STANFORD, Calif. - A little girl just a week shy of her second birthday has become the youngest person in the United States ever to receive a "piggyback" heart transplant, a procedure that involved implanting a second heart into her tiny chest.
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Although such heterotopic transplants aren't performed often, Camila's doctors say she will be able to have a normal life with two hearts, each of which maintains its own distinct rhythm. Of the eight children who received a second heart between 1997 and 2001, five are still alive and doing well.
If either of Camila's twin hearts ever fail, doctors will be able to replace them with a single new one because Camila's lungs will have returned to their proper pressure and size, said Reitz.
More:
http://www.mercurynews.com/mld/mercurynews/news/local/states/california/counties/alameda_county/9990962.htm