Original: Leopold Stokowski and Dimitri Mitropoulos were the orchestra's two conductors in 1949, with Mitropoulos becoming Musical Director in 1951.
Simplified: In 1957, Mitropoulos and Leonard Bernstein served together as Principal Conductors until, in the course of the season, Bernstein was made conductor, becoming the first American-born-and-trained conductor of the Philharmonic.