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