Coregency

   A system of dual rule that was devised to ensure the automatic transfer of power to the junior ruler upon the death of the elder. It seems to have first been employed by Amenemhat I, who made his son, Senusret I, a joint ruler, although in this case the succession was disputed. Two types of coregency are known whereby the junior partner has a full royal titulary and regnal dates or simply the titulary and no separate year dates until his succession as sole ruler. Coregencies have complicated the determination of the exact chronology of rulers as they are not always attested clearly or taken account of in the surviving king lists. Some coregencies proposed by some modern Egyptologists like that between Amenhotep III and Akhenaten are disputed.

Historical Dictionary Of Ancient Egypt by Morris L. Bierbrier