Wikipedia
The term polytempo or polytempic is used to describe music in which two or more tempi occur simultaneously.
In the Western world, the practice of polytempic music has its roots in the music theory of Henry Cowell, and the early practices of Charles Ives. Later on, composer Elliott Carter, in the fifties, began polymetric experiments in his string quartets that inevitably amounted to polytempic behavior by nature of several competing lines at different surface speeds. At around the same time, composer Henry Brant expanded on Ives Unanswered Question to create a spatial music in which entire ensembles, separated by vast distances, play in distinct simultaneous tempi.
Some types of African drumming exhibit this phenomenon.
Today's composers are employing polytempi as a compositional strategy to create total and complete independence of line in polyphonic music. Composers such as Conlon Nancarrow, David A. Jaffe, Evgeni Kostitsyn, Kyle Gann, Kenneth Jonsson, John Arrigo-Nelson, Brian Ferneyhough, Karlheinz Stockhausen, Frank Zappa, and Peter Thoegersen have used various methods in achieving polytempic effects in their music.
Polytempic music also harkens to the rhythmic practices of some Renaissance and medieval composers (see hemiola).