Complex sentence: He was principal flute of the New York Philharmonic for 18 years; in addition, he was one of the founding members of the Bach Aria Group, with whom he played from 1946 to 1964.
Split into simpler sentences: He was principal flute of the New York Philharmonic for 18 years. Additionally, he was one of the founding members of the Bach Aria Group, with whom he played from 1946 to 1964.