§1. A pastor is to have the parochial care of only one parish; nevertheless, because of a lack of priests or other circumstances, the care of several neighboring parishes can be entrusted to the same pastor.
§2. In the same parish there is to be only one pastor or moderator in accord with the norm of can. 517, §1; any contrary custom is reprobated and any contrary privilege whatsoever is revoked.
Source: Code of Canon Law (Vatican.va)