The Graduate Partnerships Program (GPP), established in 2000, links universities with National Institutes of Health (NIH) laboratories for predoctoral training. Several partnerships required that students create collaborative dissertations between at least one NIH and one university research mentor. More than 60 students have entered into these co-mentored research collaborations, and many others established them even though not required. Much was learned about the experiences of these and other GPP students by using structured interviews as part of a formal self-study of the GPP in 2005. Complications of trying to work with two mentors are managed through careful program design and mentor selection. In the collaborative model, students develop a complex set of scientific and interpersonal skills. They lead their own independent research projects, drawing on the expertise of multiple mentors and acquiring skills at negotiating everyone's interests. They develop high levels of independence, maturity, flexibility, and the ability to see research questions from different perspectives. No evidence was found that co-mentoring diminishes the normally expected accomplishments of a student during the Ph.D. Multi-mentored dissertations require skills not all graduate students may possess this early in training, but for those who do, they can promote rapid and extensive development of skills needed for collaborative, interdisciplinary research. (Contains 3 tables.)