Who was Yehoram?
Rashi: He was Achazyahu's brother.
How could Yehoram ben Achav become king in the second year of Yehoram ben Yehoshafat? Achazyahu became king in Yehoshafat's 17th year, and he reigned for two years, and Yehoshafat reigned for 25 years before his son Yehoram became king! And below (3:1) it says that Yehoram ben Achav became king in Yehoshafat's 18th year, and Yehoshafat went with him in the war against Mo'av!
Rashi (from Tosefta Sotah 12:2): Yehoshafat should have died in Ramos Gil'ad (for joining with Achav, a Rasha. In the merit that he cried to Hashem, he was given another seven years. From that time, his kingship is counted for his son. ('Yehoram ben Yehoshafat's second year' is really Yehoshafat's 18th year.')
Radak: Yehoshafat, in his lifetime, appointed his son Yehoram to be king, due to controversy with his brothers. It says "v'Es ha'Mamlachah Nasan li'Yhoram Ki Hu ha'Bechor" (Divrei ha'Yamim II, 21:3. Obviously it is to the Bechor! Rather, he made him king in his lifetime, due to controversy with his brothers. He ruled for seven years in his father's lifetime. Yehoram ben Achav became king in the second year that Yehoram ben Yehoshafat ruled in Yehoshafat's life (in the 18th year from when he became king).
