Manasseh, Ephraim; though more near and dear to one another than any other tribe, being both sons of Joseph. They together shall be against Judah; which might be accomplished either before Shalmaneser took Samaria, or afterwards. For though the Israelites were miserably destroyed at that time, yet they were not utterly rooted out; of which See Poole on "Isaiah 7:8".
Related Commentaries of Isaiah 9:21
Isaiah 9:21
21 Manasseh, Ephraim; and Ephraim, Manasseh: and they together shall be against Judah. For all this his anger is not turned away, but his hand is stretched out still.