Complex sentence: Sibelius, whose last three symphonies were given their American premieres in Philadelphia in the 1920s; and Igor Stravinsky, many of whose works were also given their first American performances by Stokowski.
Split into simpler sentences: Sibelius, whose last three symphonies were given their American premieres in Philadelphia in the 1920s. Also, Igor Stravinsky, many of whose works were also given their first American performances by Stokowski.