Manasseh and Ephraim represent the northern kingdom because they were its largest tribes. These tribes stood for the whole kingdom of Israel.
The two largest tribes of Israel and Judah fought against each other. This likely refers to the war between Judah and the alliance of Syria and Israel. This war is often called the Syro-Ephraimite War (Isaiah 7:1–12). During this conflict, people from the same family fought against one another. In other words, brothers fought against brothers (2 Chronicles 28). This civil war brought great suffering to God’s people.