Complex sentence: Gustav Mahler chose him as concertmaster for the New York Philharmonic for two years from 1909, and was called to conduct the orchestra for the final seventeen concerts of 1911 during Mahler's illness.
Split into simpler sentences: Gustav Mahler chose him as concertmaster for the New York Philharmonic 1909-1911. He conducted the orchestra for the final seventeen concerts of 1911 due to Mahler's illness.