We investigate the mass transfer properties at the evolutionary stage after the mass transfer being started, in two Double White Dwarfs binary stars – CR Boo and V 803 Cen. Both are AM CVn stars objects: short period binaries; helium-rich accreting material; The mass transfer is considered to be realized via the Roche-lobe overflow; Solutions of the mass transfer stability by the dynamical stability criteria; The accretion efficiency of two objects is estimated. The obtained values are very low.