Interestingly, if you look at the film closely it's possible De Niro and Pacino are never in the same frame at the same time (despite the shot you include in this article, which isn't in the film, as the cafe scene doesn't include that wide angle, and the entire scene could have been filmed using body doubles).

I mentioned this to a friend during the original release (we both saw the film twice) and his theory was that putting both actors onscreen at the same time would create a burning excess of quality that would cause the celluloid to melt (which is why they were in separate scenes in The Godfather Part II). Of course, they've been onscreen together since, so my friend's theory has since been debunked. :)