1)
When does this discuss?
1.
Radak, Malbim: It is when the exiles will be gathered.
2)
What does it mean that Beis Yehudah will go "Al Beis Yisrael"?
1.
Rashi: They will join with them 1 and be added to them to be one kingdom.
2.
Malbim: They will join, make one leader and ascend to their fathers' land.
1
Radak: This is like "va'Yavo'u ha'Anashim Al ha'Nashim"(Shemos 35:22).
3)
Why does it say "me'Eretz Tzafon"?
1.
Radak: This is like I wrote (verse 12) that the [10] tribes were [exiled] in the north, to the east, past Bavel.