The Pharaoh also did this when he was elderly or unable to rule his country on his own (such as the case of Thutmose III and Amenhotep II or Amenemhat I and Senusret I).
This begins one-year coregency sometime in the six months on or after Nisan 1 of 842 BC, which was in the 11th year of Jehoram of Israel (2 Kings 9:29) by Israel's Nisan calendar and non-accession reckoning (2 Kings 9:29).
An analogous situation of two ways of measurement, both equally valid, is encountered in the dates given for Jehoram of Israel, whose first year is synchronized to the 18th year of the sole reign of Jehoshaphat of Judah in 2 Kings 3:1 (853/852 BC), but his reign is also reckoned according to another method as starting in the second year of the coregency of Jehoshaphat and his son Jehoram of Judah (2 Kings 1:17); both methods refer to the same calendrical year.