you arent allowed to get out of the exit room there is a cube thing in the exit roiom where you have to place the cube

Well yeah, what if you get the companion cube OUT of the room? You place companion cube on the button (flipping portallable-surfaces). You stand on the button which opens your way to the exit room. Your friend goes to the exit. You place portal on the ceiling of the corridor (leading to the exit room). You place second portal on the flipped surface (remember, companion cube is out of the exit room). You go through portal, fall from the ceiling. And here we go - no way to get out.

I know it's a little bit complicated to describe, but I did my best

well i played it and i didnt find any errors did you place it on the right button? and you dont have to get back for the cube because there is a companion cube at the exit

There is a "dead-lock" situation, if you place a cube on the button which flips portallable-surfaces (room with 2 turrets), and you and your team-mate get to the room leading to exit: without a cube and without a possibility to get back for it. Fix it!