Complex sentence: Two other professional musicians were also among his fellow prisoners (violinist Jean le Boulaire and cellist Étienne Pasquier), and Messiaen wrote a short trio for them; this piece developed into the "Quatuor" for the same trio plus himself at the piano.
Split into simpler sentences: Two other professional musicians were also among his fellow prisoners (the violinist Jean le Boulaire, and the cellist Étienne Pasquier), and he wrote a short trio for them. This piece developed into the "Quatuor" for the same trio, plus himself at the piano.