question: Did M1 star a person 's actor and star M2
query: SELECT count(*) WHERE {
  ?x0 ns:film.actor.film/ns:film.performance.character ?x1 .
  ?x1 a ns:people.person .
  M1 ns:film.film.starring/ns:film.performance.actor ?x0 .
  M1 ns:film.film.starring/ns:film.performance.actor M2
  }