This question has been discussed in Why was the Triwizard cup portkey in Goblet of Fire two-way? and Why Did the Triwizard Portkey Return Harry to the Outer Edge of the Maze?.
In short, the portkey already contained the charm that would teleport the winner (the first person who touched it) to the beginning of the maze — this was by design of the tournament. Note that only one such portal existed, because there was only one cup, to be held by the only winner (Harry and Cedric weren't expected to touch it together).
It's possible that Barty Crouch Jr. couldn't remove that charm (or didn't bother to, or it'd alert someone), so he put the charm that teleported Cedric and Harry to the graveyard on top of the existing charm.
Thus we have two layers: the outer layer (put by Crouch Jr.) teleports you to the graveyard; the inner layer (by design of the tournament) teleports you to the beginning of the maze.