Why are both Joseph and Manasseh listed as separate tribes in Revelation chapter seven? The Patriarch Joseph had two sons Manasseh, and Ephraim:


  Genesis 46:20  KJV  And unto Joseph in the land of Egypt were born Manasseh and Ephraim, which Asenath the daughter of Potipherah priest of On bare unto him. 


Israel gave Joseph an extra share among his brothers in:


  Genesis 48:22  KJV  Moreover I have given to thee one portion above thy brethren, which I took out of the hand of the Amorite with my sword and with my bow. 


Both Manasseh and Ephraim were given equal shares of the promised land, when it was divided among the tribes. Manasseh also was given an area outside the promised land.


  Joshua 14:4  KJV  For the children of Joseph were two tribes, Manasseh and Ephraim: therefore they gave no part unto the Levites in the land, save cities to dwell in, with their suburbs for their cattle and for their substance. 
  
  Joshua 22:7  KJV  Now to the one half of the tribe of Manasseh Moses had given possession in Bashan: but unto the other half thereof gave Joshua among their brethren on this side Jordan westward. And when Joshua sent them away also unto their tents, then he blessed them, 


In the Revelation chapter 7 Manasseh and Joseph are listed as tribes, but Ephraim is not.

Rev 7:4 through 8     KJV  


  And I heard the number of them which were sealed: and there were sealed an hundred and forty and four thousand of all the tribes of the children of Israel. Of the tribe of Juda were sealed twelve thousand. Of the tribe of Reuben were sealed twelve thousand. Of the tribe of Gad were sealed twelve thousand. Of the tribe of *Aser***were sealed twelve thousand. Of the tribe of ***Nepthalim* were sealed twelve thousand. Of the tribe of *Manasses ***were sealed twelve thousand. Of the tribe of ***Simeon* were sealed twelve thousand. Of the tribe of Levi were sealed twelve thousand. Of the tribe of Issachar were sealed twelve thousand. Of the tribe of Zabulon were sealed twelve thousand. Of the tribe of Joseph were sealed twelve thousand. Of the tribe of Benjamin were sealed twelve thousand. 


Is the Tribe of Joseph supposed to be Ephraim?
There are actually 14 tribes if you consider Ephraim and Manasseh as additional tribes, but this is an abberation in my opinion since Ephraim inherited his father's portion so there were 13.  Levi was not given land, his offspring being the priests.

As you indicated Joseph received extra share of land which I suspect is what allowed Manasseh to separate from Ephraim as a 13th tribe.  A tribe is essentially a group of people who live together on a single plot of land so by logic if you separate into another plot of land you naturally create a new tribe.

Ephraim however was the oldest and inherited his father's share.  Although the oldest son inherited his father's house it was still his father's house and everything bestowed on that house also belonged to the father as long as he was alive (the house was never divided).

Most people today don't realize that.  Being the oldest was a mixed blessing.  Sure you inherited everything - but that included accountability and responsibility for the house and it's occupants, and deference to his father as long as the father lived.

Interestingly Dan is also missing from Revelations (Ephraim is represented via the mention of Joseph - imho).  This makes the count in Revelations back to 12 tribes.  Why is Dan not mentioned is a more intriguing question to me.