Bach: an illusion of two violins
There is a concept in auditory scene analysis (how we separate and group all the sound sources we might hear at once, like the cocktail party effect) called “auditory streaming”. It just means that we will tend to group sounds of similar pitch into the same perceived object. For example, I could play you a trill on a piano (so, two notes not separated much in pitch), and you would hear it all as one stream. If instead I played two notes separated much more in pitch, you would interpret that as two different streams.
Bach uses this idea in his Prelude for Partita Number 3 for solo violin, to create the illusion of multiple violins. Below is a video of Jascha Heifetz playing the Prelude (http://www.youtube.com/watch?v=ruu1JqRPPic). There are many points during the piece when the violinist bounces back and forth between low and high pitches so quickly that it sounds like two violins are playing. Best places in the video to hear this: 2:30-2:43, 2:59-3:03, and 2:08-2:13.