The "alternate realities" idea was done in fiction by Borges in his story|"The Garden of Forking Paths", and the Australian composer Richard Meale wrote a big orchestral piece structured after it - the idea of parallel aleatorism was used by several composers in the 60s. You get other versions of it in Bernd Alois Zimmermann's "Die Soldaten" and in the SF of Philip K. Dick (set to music by Tod Machover in "Valis", a piece I haven't heard).

Meale's piece was quite early and I don't think he ever wrote anything as interesting afterwards.