Despite the similarities in basic definition a movie duo and a movie couple are NOT the same thing. Yes, it's a relationship shared between two people, but the key difference is the element of romance.
Movie duos have no romantic involvement, in fact, they don't even really need to like one another, let alone love. Take Ed Norton and Brad Pitt in Fight Club as an example—considering by the film's end, we find out that both characters are Tyler Durden, this may become a bit difficult to explain, but essentially both Pitt and Norton had a symbiotic throughout the entire movie. One did not exist with out the other, at least as far as the overall plot is concerned. That's exactly what a movie duo is—one character, not only strengthening, but sharing the overarching narrative.
This fact is consistent regardless of genre—comedy, horror, action, drama—it's irrelevant, a duo is a duo! I happen to prefer duos in drama like The Shawshank Redemption or Good Will Hunting, but that's not to say that they aren't great all around. Of course, comedic duos have been a staple in the film industry for quite some time, if anything I'd venture to say that's most of what you guys will recognize below!