Prior to stem cell infusion, participants will receive a preparative regimen of total lymphoid irradiation (TLI), fludarabine, cyclophosphamide, melphalan, and thiotepa to prepare their bone marrow. Thereafter, they will receive a hematopoietic cell graft from a haploidentical donor and an unrelated umbilical cord blood donor. Post-transplantation immunosuppressive treatment will include tacrolimus and mycophenolate mofetil.