Do you know music for piano where the two hands don't have to play in the same key?
The term for such music is polytonality.
Keep in mind that polytonality may not necessarily apply to the entire composition. It would not be un-common for a composition to have only parts which are polytonal.
Many polytonal works will be from the early 20th century. You will see some of the composer names in the wiki article: Stravinsky, Bartok, Milhaud, Ives, etc.
Here is one the cited examples from Milhaud: Copacabana. Right hand is in
B major and the left hand is
Some things to keep in mind:
- Each part separately is pretty clear and simple (diatonic within their respective tonalities.)
- The listener should be able to hear each part is in a separate key. From examples I have seen, that separation is largely contingent on the clarity of the separate parts. You should need to strain to hear the two keys.
- Some counterpoint/rhythmic contrast between parts will help with perceiving the separation.
- A large tonal contrast help with separation. The difference between
G(one sharp) and
B(5 sharps) is 4 accidentals. That's a pretty large contrast of tones. It's hard for
Gto be confused with
B. But, compare
Fmajor. The difference is only 1 flat. When combining those two keys the results could sound like extended harmonies of 9th or 11th chords, or perhaps a
Ctonic with a variable natural
b^7scale degree. A similar point could be made about blues tonality. It isn't polytonal, it's a idiomatic blending of major and minor and 'blue' notes, but clearly there is only one tonic.
That's probably the essential point with polytonality: you really need to hear two (or more?) simultaneous tonics.