<P> Polytonality ( also polyharmony ( Cole and Schwartz 2012 ) ) is the musical use of more than one key simultaneously . Bitonality is the use of only two different keys at the same time . Polyvalence is the use of more than one harmonic function , from the same key , at the same time ( Leeuw 2005 , 87 ) . </P>
the technique of using two or more tonal centers at the same time is called
Polyvalence