How can it be the fifth year of Yehoram ben Achav and of Yehoshafat? Yehoram became king in Yehoshafat's 18th year (3:1)!
Rashi citing Seder Olam 17: It was Yehoshafat's fifth year 1 from when it was decreed that he die in Ramos Gil'ad (Radak - for going to help the Rasha Achav). Because he cried to Hashem, he was given another seven years. He made his son Yehoram king in his lifetime, two years before he died.
Radak: "Vi'Yehoshafat" does not mean that it was the fifth year also of Yehoshafat. Rather, that is when Yehoshafat died. His son Yehoram then ruled a second time, for he already began ruling seven years earlier. Now he ruled without any contender; he killed his brothers (Divrei ha'Yamim II, 21:4). Even though the word "Mes" is omitted, it is understood; many verses omit words that are understood. 2 Since Yehoshafat's son ruled, it is understood that Yehoshafat died.
Ralbag: Because Yehoshafat married his son to the daughter of Achav Melech Yisrael, he helped to run Malchus Yisrael. They honored him because he was old and married into Achav's family. The verse considers it as if he and Yehoram ben Achav ruled together. Malbim - Divrei ha'Yamim II, 19:4 implies that Yehoshafat ran Yisrael - "va'Yashav v'Yetzei va'Am mi'Be'er-Sheva Ad Har Efrayim va'Yshivem El Hashem" - [Har Efrayim] was in Malchus Efrayim! Yehoram ben Achav removed his father's Matzevas ha'Ba'al; this caused his partnership with Yehoshafat.
Radak rejects this (refer to 8:16:2:1**).
Radak: E.g. (Mishlei 28:16) "Nagid Chaser Tevunos v'Rav Ma'ashakos Sonei Vetza Ya'arich Yamim" is missing the word Yamus (before "Sonei"). The Nagid (leader) without understanding and much theft, he will die, but the one who hates unjust gain, will live long.
How can it say that Yehoram ben Yehoshafat became king in the fifth year of Yehoram ben Achav? Above (1:17), it says that Yehoram ben Achav became king in Yehoram ben Yehoshafat's second year!
Radak citing Seder Olam 17: The verse above counts it as Yehoram ben Yehoshafat's second year, for Yehoshafat should have died and it should have been Yehoram ben Yehoshafat's second year. 1 Really, Yehoshafat made his son Yehoram king only five years later 2 , two years before he died.
Refer to 8:16:1:2,3.
Malbim: When Yehoshafat returned from Ramos Gil'ad, he partially made his son Yehoram king, like it says in Divrei ha'Yamim. (I do not know from which verse he learns. - PF) The next five years are counted for both of them. After the five years, he absolutely made Yehoram king.
Radak: How can this be? Achazyahu ruled for two years after Achav died (in Ramos Gil'ad). If Yehoram ben Yehoshafat is considered king from then, his seventh year is the fifth year of Yehoram ben Achav, who became king after Achazyahu died! Also, Seder Olam says that they made Yehoram ben Yehoshafat king after killing Amon, Mo'av and [the residents of] Har Se'ir. This was in Achazyahu's lifetime! (Divrei ha'Yamim II, 20:22-24 discusses their death, and verse 35 says that afterwards, Yehoshafat joined with Achazyahu.)
