How could he marry two wives? A Kohen Gadol may have only one wife (Yuma 13a)!
Malbim: He married one after the other [died or was divorced]. Even though he was 90 years old, he married Na'aros Besulos, for a Kohen Gadol may not marry a Bogeres. They died in his lifetime and he fathered more children ? "Od Yenuvun b'Seivah" (Tehilim 92:15).
Malbim (Divrei ha'Yamim I, 5:30): Seder Olam says that also Pedayah was Kohen Gadol in the days of Yo'ash. Yehoyada married a second wife after he ceased serving as Kohen Gadol 1 , and was merely Kohen ha'Rosh (refer to Divrei ha'Yamim I, 5:36:3:4).
Due to weakness in his old age. A Kohen Gadol may have two wives, just when he serves on Yom Kipur he must have only one wife - "v'Chiper? u'Ve'ad Beiso." Amidst concern lest his wife die, he would marry a second wife, and divorce her conditionally (Yuma 13b). (PF)
Why does it say that Yehoyada had children?
'Rashi': Below (verses 22, 25) it says that Yo'ash killed his sons, so it needed to teach beforehand that he had sons.
