Complex sentence: Harris brought Schuman to the attention of the conductor Serge Koussevitzky, who championed many of his works and conducted Schuman's Symphony No.2 in 1939.
Split into simpler sentences: Harris brought Schuman to the attention of the conductor Serge Koussevitzky, who championed many of his works. Koussevitzky conducted Schuman's Symphony No.2 in 1939.