Let me explain better what I'm talking about.
Gen 48:5-6
5 "Now then, your two sons born to you in Egypt before I came to you here will be reckoned as mine; Ephraim and Manasseh will be mine, just as Reuben and Simeon are mine.
6 Any children born to you after them will be yours; in the territory they inherit they will be reckoned under the names of their brothers.
(NIV)
This suggests to me that Ephraim and Manasseh are to be regarded as sons of Jacob, not of Joseph, and that any other children Joseph has will be regarded as his own. This suggests to me that there could be a tribe of Joseph, a tribe of Ephraim, and a tribe of Manasseh.
48:6 isn't particularly clear to me. I think it means that Josephs sons will inherit the same land as Ephraim and Manasseh.
Gen 48:22
22 And to you, as one who is over your brothers, I give the ridge of land I took from the Amorites with my sword and my bow."
(NIV)
John 4:5
5 So he came to a town in Samaria called Sychar, near the plot of ground Jacob had given to his son Joseph.
(NIV)
This suggests to me that Jacob is giving Joseph, and presumably Josephs sons, after Ephraim and Manasseh, a specific piece of land.
1 Chr 7:20-24
20 The descendants of Ephraim: Shuthelah, Bered his son, Tahath his son, Eleadah his son, Tahath his son,
21 Zabad his son and Shuthelah his son. Ezer and Elead were killed by the native-born men of Gath, when they went down to seize their livestock.
22 Their father Ephraim mourned for them many days, and his relatives came to comfort him.
23 Then he lay with his wife again, and she became pregnant and gave birth to a son. He named him Beriah, because there had been misfortune in his family.
24 His daughter was Sheerah, who built Lower and Upper Beth Horon as well as Uzzen Sheerah.
(NIV)
"Here we are told that two sons of Ephraim died as cattle rustlers down near gath, and that "their father Ephraim mourned for them many days." Then it tells of Ephraim's having another son, and a daughter who proceeded to found three settlements in the region. All this seems to suggest a scenario in which Ephraim is living in Canaan rather then in Egypt. Now add to this the fact that at the time of Joshua's conquest, himself of the tribe of Ephraim, there is no record of a conquest in central Canaan in and around Shechem, but only of northern and southern campaigns. Nevertheless, it was to Shechem that Joshua mustered the tribes before his death to challenge them to be faithful to Yahweh and to renew the covenant." - Dictionary of the Old Testament Pentateuch - Joseph.