Complex sentence: She was the first female ballet master of the Royal Ballet, though she did not hold this position alone but shared it jointly with P. E. Wallquist, and left her position before him to become the principal of the ballet school.
Split into simpler sentences: She was the first female ballet master of the Royal Ballet, though she did not hold this position alone but shared it jointly with P. E. Wallquist. She left her position before him to become the principal of the ballet school.