We present a synergic experimental and theoretical investigation of the plastic relaxation onset in low-temperature Ge growth on Si(001). High-resolution transmission electron microscopy reveals that misfit is released by pairs of coupled 60 degrees. dislocations. Atomic resolution proved to be key in distinguishing pairs from single 90 degrees. dislocations because of the revealed small intrapair dislocation distance (even less than 1 nm). By exploiting dislocation theory and molecular dynamics simulations, we demonstrate that the observed pairing naturally occurs as a result of the mutual interactions between the two dislocations. In particular, analytical models show that the stress field arising in a thin film when a dislocation segment lies at the interface with the substrate determines the most favored nucleation site for a new (complementary) dislocation that leads, after migration, to the coupling with the first in a stable position. At the growth temperature, further motion or recombination due to atomic scale effects is excluded by classical molecular dynamics simulations. A clear picture of the early stages in the strain relaxation emerges, gliding out of the interface and/or short-range climbing (as typically produced by annealing or higher temperature steps but not taking place under the present growth conditions) being required to transform pairs into edge dislocations. The present results also offer answers to the long-asked puzzling question about the mechanism originating 90 degrees. dislocation in high-mismatch Ge/Si systems.