One that comes to mind (although it is piano duet) is Holst, "The Planets" suite. Most people know it as an orchestration only but little do they know that it was first a piano duet to begin with.
But with the last section ("Neptune the Mystic") originally set for organ, Holst considering the music too delicate for a percussive instrument as the piano.I think I once read that the autograph score of the orchestration (with some exception, Mars?) is not in Holst's own hand. Does anyone know more about this?All best,gep