The effects of feasibility oExperimental design and methods f allo-allo tandem matched stem cell transplantation (AATT) in patients with refractory leukemia will be evaluated in a clinical setting. The current study is limited only for patients with refractory disease that received and failed up to 2 lines of salvage therapy, in good performance status and younger than 50 years old. Only patients that will achieve complete remission after transplant I, will have no major organ dysfunction and with acceptable performance status, will be treated with transplant II. Close monitoring with strict stopping rules including in case of excess transplant related morality, acute or chronic GVHD or graft failure will be employed.