Joker was essentially a remake of The King of Comedy with DC characters shoved in.
It worked because The King of Comedy is a great film, and especially because of Joaquin Phoenix's masterful performance.
I have no idea what Folie à Deux will be based on (New York New York, maybe..?), but it looks like it'll be quite a different film from Joker (a tragic romance musical, it seems) while probably at the same time trying to touch similar themes... which would make hitting the right spot (again) quite more difficult than even for the first film...
Personally I don't have much hope and expect it will end up being a disappointment (and even if it ends up being as good as the first one it might be a box office flop due to the original audience not being into romantic musicals), but I'd love to be proven wrong.