Simplify the following text for a Grade 10 native English-speaking person.
Original: Leopold Stokowski and Dimitri Mitropoulos were appointed co-principal conductors in 1949 , with Mitropoulos becoming Musical Director in 1951 .
Simplified: Leopold Stokowski and Dimitri Mitropoulos were the orchestraâ s two conductors in 1949 , with Mitropoulos becoming conductor in 1951 .