Albert Barnes Commentary 2 Kings 14:2

"He was twenty and five years old when he began to reign; and he reigned twenty and nine years in Jerusalem: and his mother`s name was Jehoaddin of Jerusalem." — 2 Kings 14:2 (ASV)

Joash of Judah reigned for forty years (2 Kings 12:1), and Joash of Israel ascended the throne in his namesake’s thirty-seventh year (2 Kings 13:10). Therefore, we would expect Amaziah to have succeeded his father in the fourth, rather than in the second, year of Joash of Israel. The usual explanation for this discrepancy is to suppose a double accession of the Israelite Joash—as co-regent with his father in the thirty-seventh year of his namesake, and as sole king two years afterward.