First Time Home Buyer Credit?

Tenants in Common: Is person B eligible for the first-time home buyer credit when person B buys a portion of the ownership interest in an existing house that person A has owned, and subsequently, person A and person B become tenants in common. Person A would not be eligible for the credit. However, since person B is "purchasing" their principal residence, and being put on the Mortgage Title, is this person eligible for their ownership portion (ume 50 / 50 split)?