In French Suite II, Air (mm. 3) it sounds like the upper voice can be separated into two parts: the upper part being Eb up to G, down to F; and the lower part being C down to Bb, down to Ab.
In counterpoint, is there a name for when a single voice can serve the function of multiple voices?
Although the terms discussed in Aaron's answer make sense to me, I've heard the term "compound melody" used much more frequently. In short, the term suggests that a single-line melody can be understood as a fleshing out of multiple different lines, hence the term "compound."
"Compound melody" largely gained popularity as a Schenkerian concept to explain—exactly as you were seeking—polyphony in Bach. But these days it's largely been stripped of that baggage.
It's called "implied polyphony" or "single-voice polyphony". The solo string works of Bach are excellent examples, and they function similarly to the instance being asked about here. They typically involve leaps in register that break up the sense of one voice. Bobby McFerrin is famous for doing this vocally, creating multiple voice parts by jumping from one register to another. (This is done solo, live, as opposed to his overdubbed recordings.)
A Bach example, his first suite for solo cello, performed by Pablo Casals, who "discovered" them (they'd been forgotten until he came across them):
And here is some implied polyphony by Bobby McFerrin: The Beatles's "Blackbird". This isn't counterpoint, per se, but it certainly illustrates how multiple voices can be implied.
