This appears to be another mistake.
How did the same Jehoram die from an arrow in the heart at the same time he died from a bowel disease given to him by God?
2Ki 9:24 And Jehu drew a bow with his full strength, and smote Jehoram between his arms, and the arrow went out at his heart, and he sunk down in his chariot.
2Ch 21:19And it came to pass, that in process of time, after the end of two years, his (Jehoram) bowels fell out by reason of his sickness: so he died of sored iseases. And his people made no burning for him, like the burning of his fathers.
This one is easy to explain (it wasn't the same person), but understandable, as Israel and Judah each had both a Jehoram (aka Joram) and an Ahaziah as kings. You've mixed up Jehoram of Israel, who was killed by Jehu's arrow in 2 Kings 9:24, with Jehoram of Judah, who died of a bowel disease. Jehoram followed Ahaziah (son of Ahab) as king of Israel and was contemporary with both Jehoram (son of Jehoshaphat) and Ahaziah (son of Jehoram) of Judah.