Joy Hindle cried when she found out she couldn't give one of her kidneys to her twin brother. Then doctors gave the Bel Air, MD, woman another option: a kidney exchange, in which she would donate her kidney to a patient who needed one, and her diabetic brother would get one from another willing donor. Hindle and brother Paul McSorley were two of six participants in a triple kidney transplant performed last month by doctors at the University of Maryland Medical Center. She gave her kidney to a stranger; he got a kidney from a stranger. Multipatient swaps made up a small percentage of the more than 13,000 kidney transplants performed in the United States last year. But with the creation of a national database to link the 90,000 patients waiting for kidneys with compatible donors, some health care officials expect the number to increase dramatically.