There are two problems with that sentence; first, “the couple” is a singular object, so the verb should be “flies” not “fly” (Bob flies, Bob and Fred fly). So the immediate reaction is to change the line to “the couple still flies separately.”
But that doesn’t account for the second problem; “the couple” — as a singular object — cannot fly “separately” because, well, it’s a single thing. So it’s not just a grammatical issue; there’s a conceptual mistake.
Vanity Fair, as far as I can tell, has high editorial standards, so how could this double-whammy get through? In seeking the rationale for the first problem (fly vs. flies) I thought, “What would Bill Walsh do?” (If you have any interest in editorial machinations in a context that is generally free of the polarizing descriptive vs. prescriptive arguments, you should read Bill’s blog and his web site. Bill flies no flags, he just makes sense.)
Then, as I brushed my teeth, it came to me. “The couple,” in this sentence, is shorthand for “the members of the couple.” So in fact, it is a plural, not singular. The “error” is in not spelling it out, but the editorial argument is (probably) that doing so is unnecessarily awkward, and in the context of the paragraph, the context of “the couple” is obvious. Note that this interpretation solves both problems.
Prescriptivists (of which I am not but am often accused of being) will reject that position, and the descriptivists (whom I have been accused of disliking, when in fact I often side with them) have already stopped reading this post because they never saw a problem in the first place.
But what I’m interested in is the editorial position. Personally, I would have re-cast the sentence as “Mendez and Winslet still fly separately,” or simply “They still fly separately,” but it depends on how the rest of the paragraph is cast.
However, I now understand the choice of “fly” over “flies” even if I don’t fully agree with it. And now I will move on to the next thing.
And so passes a Sunday morning chez nous.