Albert Barnes Commentary 2 Kings 1:17

"So he died according to the word of Jehovah which Elijah had spoken. And Jehoram began to reign in his stead in the second year of Jehoram the son of Jehoshaphat king of Judah; because he had no son." — 2 Kings 1:17 (ASV)

The similarity of names in the royal houses of Israel and Judah at this time, and no other, appears to be a consequence of the close ties that united the two reigning families. This is well noted among the “undesigned coincidences” of the Old Testament.

According to 2 Kings 3:1, the accession of the Israelite Jehoram (Ahab’s brother) took place in the eighteenth year of Jehoshaphat. Jehoram of Judah, however, perhaps received the royal title from his father as early as his father’s sixteenth year, when he was about to join Ahab against the Syrians. The same year could therefore be called either the eighteenth year of Jehoshaphat or the second year of Jehoram.