Crimp

[Jessica]

is a song sung between two or more people (though very difficult with more than two people), which unlike a duet is completely unpractices and is made up on the spot, with two minds so completely in sync they work as one, practiced by Noel Fieldings and Julian Barratt the mighty Boosh on BBC3

[Hina Muqadas]

The act of making up a random song on the spot and yet being completely in sync with another person or persons.