A model of the elementary step motion on a two-domain (100) silicon surface during crystallization from a molecular beam is proposed. The model takes into account the possibility of an adatom transition to an adjacent terrace before incorporation into a kink at the A-step edge (the effect of the A-step permeability). It is shown that the permeability of the A-step contributes to the faster pairing of the A- and B-steps and, consequently, transition to a single-domain surface. For the fast pairing of the steps, it is sufficient only the presence of an inverse Ehrlich–Schwoebel barrier for the attachment of adatoms to the A-step from the B-type terrace. A conventional barrier (for the attachment from the A-type terrace) may be absent, which is consistent with the results of quantum chemical calculations.