On a very literal level, Duncan and Lorraine are just two old buddies of Charlie's who want to hang with their old friend while he's in town. They're peeved that he keeps blowing them off because, hey, friends shouldn't act that way. From their perspective, they wonder what's wrong with going out for a drink?
On a more allegorical level, Duncan and Lorraine represent the past that Charlie has left behind. While he is making attempts to turn his life around and grow up, the two of them are doing exactly the same thing they were years ago: drinking and partying. When the two of them pursue Charlie, then, it's as though his past is pursuing him all over Paris, tempting him to rejoin their shenanigans.
Fitzgerald seems to encourage this darker interpretation of Lorraine and Duncan with a few key lines. When the two first find Charlie in the restaurant with his daughter, he thinks of them as "ghosts from the past" (2.41). We feel the temptation they represent to Charlie when he feels "Lorraine's passionate, provocative attraction" (2.44).
And we definitely know something is up when we're told that "they liked [Charlie] because he was functioning, because he was serious; they wanted to see him, because he was stronger than they were now, because they wanted to draw a certain sustenance from his strength" (2.57). This is powerful characterization, as it makes Duncan and Lorraine seem almost parasitic without explicitly saying anything negative about them. We fear for Charlie because this makes it sound as though the pair want to take something from him. They don't just want his company; they want to feed off of his new-found resolve. Yikes, right?