Complex sentence: The singers were Gro Anita Schønn and Solfrid Heier, who was substituted by Toril Støa and Benny Borg on various occasions.
Split into simpler sentences: The singers were Gro Anita Schønn and Solfrid Heier. The latter was substituted by Toril Støa and Benny Borg on various occasions.