I'm afraid I don't really have an answer for this just yet (except anything with Jimmy Stewart is going to be tops in my book).
That being said, I've been giving it some thought, and while my gut reaction was "No, let's free up the list for new blood," I've come around. What made me change my mind is I thought about the greatest songs of the 20th Century and a lot of them are done by collaborators. If I were looking at a list of the top 100 songs of the 20th Century and five of them were Lennon/McCartney, compositions, I wouldn't be calling for one of them to be removed. I would be like, "That's all?"
So, yeah, I agree. If two people find themselves to be effective collaborators and bring out the best in each other, then absolutely let them stay on the list. My only caveat would be if the movies/songs were too alike. Then I would say lump them all together like Star Wars and LotR. However, I think Raging Bull, Taxi Driver, and Goodfellas are dissimilar enough that that's not really an issue here.