We have performed systematic measurements of the splitting kinetics induced by H-only and He + H sequential ion implantation into relaxed Si _0.8 Ge _0.2 layers and compared them with the data obtained in Si. For H-only implants, Si splits faster than Si _0.8 Ge _0.2 . Sequential ion implantation leads to faster splitting kinetics than H-only in both materials and is faster in Si _0.8 Ge _0.2 than in Si. We have performed secondary ion mass spectrometry, Rutherford backscattering spectroscopy in channeling mode, and transmission electron microscopy analyses to elucidate the physical mechanisms involved in these splitting phenomena. The data are discussed in the framework of a simple phenomenological model in which vacancies play an important role.