“Joker: Folie a Deux,” a dark new musical Batman spinoff, earned an estimated $40 million over the weekend to top the North American box office, industry watchers reported Sunday. But that was far behind the $96.2 opening of the original “Joker” in 2019, a movie that earned Joaquin Phoenix a best-actor Oscar and grossed more than $1 billion worldwide. Industry analyst David A. Gross called it “a weak opening for the follow-up sequel in a superhero series.”