Complex sentence: Bernard Haitink made his debut with the Concertgebouw Orchestra on November 7,1956, and after van Beinum's death, became first conductor in September 1959.
Split into simpler sentences: Bernard Haitink made his debut with the Concertgebouw Orchestra on November 7,1956. After van Beinum's death, Haitink became the orchestra's first conductor in September 1959.