Noun
coregency (plural coregencies)
A monarchical position (such as king, queen, emperor or empress) held by two persons when normally held by only one.
Traditionally held in the thirtieth year of the Pharaoh's reign, this possibly was a festival in honour of Amenhotep III, whom some Egyptologists think had a coregency with his son Amenhotep IV of two to twelve years. Source: Internet
It may also have coincided with the death of his father and the end of the coregency. Source: Internet