In Symbols of Sacred Science, René Guenon writes:

We are talking about the Two-faced Janus, a mythological figure who, in this text, refers us to the elusiveness of music. Perhaps, however, when Katia and Marièlle Labèque play together, the third invisible face of Janus can be seen, so sharp, transparent and clear are their interpretations, especially if they are dedicated to them, as is the case of Philip Glass' Double concerto for two pianos and orchestra, which turns exactly 10 years old this year, which perfectly matches the Fourth Symphony by Brahms, a masterpiece of orchestral lyricism conducted for the occasion by Music Director Emmanuel Tjeknavorian alongside Beethoven's Overture The Creatures of Prometheus.