Manasseh, Ephraim; and Ephraim, Manasseh,.... That is, "Manasseh" shall eat or devour "Ephraim"; and "Ephraim" shall eat or devour "Manasseh"; as the Septuagint, Syriac, and Arabic versions render it; which is to be understood of their quarrels, contentions, and wars among themselves, whereby they bit, devoured, and consumed each other, though they were brethren; which explains and confirms what is before said, of no man sparing his brother, and everyone eating the flesh of his own arm.
The Targum paraphrases the words thus,"they of the house of "Manasseh", with those of the house of "Ephraim", and they of the house of "Ephraim", with those of the house of "Manasseh", shall be joined together as one, to come against them of the house of Judah;''and so Jarchi interprets them,""Manasseh" shall be joined with "Ephraim", and "Ephraim" shall be joined with "Manasseh", and they together shall be joined against Judah;''so it follows,[and] they together [shall be] against Judah; as the ten tribes did sometimes make war against the two tribes of Judah and Benjamin, see 2Ch 28:6:for all this his anger is not turned away, but his hand [is] stretched out still; more and sorer judgments were to come upon this people for their sins.
See Gill on "Isa 9:12".