I strongly remember that not all flights had to be the same. It seems from previous posts here that the return also needs to be on the same flight. I thought that was possible. However, I am somehow convinced that the last feeder flight can go to different places - and that the companion could join a ZRH-DUS-JFK-DUS-ZRH pax for just DUS-JFK-DUS as well. Am not sure if this companion could do e.g.MUC-DUS-JFK-DUS-MUC however.