Albert Barnes Commentary 2 Chronicles 24:3

"And Jehoiada took for him two wives; and he begat sons and daughters." — 2 Chronicles 24:3 (ASV)

Athaliah’s destruction of the royal line had left Joash without a natural successor, and his marriage at the earliest suitable age was therefore a matter of state policy. One of his wives was probably “Jehoaddan of Jerusalem,” the mother of Amaziah (2 Chronicles 25:1), whom Joash must have married as early as his 21st year.