Complex sentence: Nelson Rockefeller, the primary backer of the Brazil project, left its board of directors, and Welles's principal sponsor at RKO, studio president George Schaefer, resigned.
Split into simpler sentences: Nelson Rockefeller, the most significant backer of the Brazil project, left the RKO board of directors. Around that time, the principal sponsor of Welles at RKO, studio president George Schaefer, resigned.