Director talking about Folie a Deux: The first movie was not about the real Joker, that is why we kill him in the second movie.
The title of the first movie, literally: JOKER
You can't make this shit up!!
And if this movie is not about the Real Joker... the one from the Batman universe, then why did he meet up with Bruce then?
Why does Arthur fall in love with Harley Quinn? Why does Arthur become literally called the Joker and Clowns pop up to be like his goons or at least spread chaos in GOTHAM ???
The idea that more people wore the name Joker exists in the comics as well but its certainly not fitting for this story. The worst part about this second movie is that it kills the idea at the end of the first one that it might only happened in Arthurs head.
From this point you could such cool things like Arthur really just being insane with now a split personality evolving because his dreams to be seen and loved by the people just like in his dream/mania as the Joker. Then he meets Harley and she is hooked by this other guy and Arthur would behave like Edward Norton in Primal Fear maybe.
The idea that the Joker is like toxin and everyone can just have one bad day to snap has been present one way or the other but Todd doesn't know why these ideas work. They work because Batman is around. For him these things matter because the Joker is his duty in every form of his insanity. Gotham at that point doesn't care. They see the persona and thats it. What changes that for the story and the world created?