Mr. Peabody Says:
Holst wrote a two piano transcription of his own music, but for me it does not quite work. My impression is that the sound of all of those instruments somehow has be expressed in the spirit of the music, and that means changing some things. The idea is to carry through the moods, textures, dynamics, overall feelings, which is hard.
So to make this come alive I added some things, changed some things. For instance, where there are just long half and whole notes, I lightly repeated chords. I also made different choices of notes at times, since it is absolutely impossible to play everything written in a symphonic score. It’s a delicate thing.
Liszt was the master:
I learned how to do this from studying what Liszt did with his Beethoven symphony transcriptions. In his time most people could not usually hear those symphonies, so they had to study the music from two piano versions, but Liszt made it all happen on one piano. He subtly changed many things, but in ways it actually made the music sound more like the full orchestral versions, not less. He was a magician.