This post holds an implied SPOILER regarding Joker: Foile a Deux, but I suppose such isn't a big deal, since the movie's polarizing ending (and all speculative paths thereof) have been well publicized, discussed and debated since the sequel's release.
I'm not dismissing the idea of one Joker begetting another (in particular, if there's a movement tied to the initial instigator), though to say there's a direct, cause-and-effect to Heath Ledger's Dark Knight version seems slim within Foile a Deux, not only due to the presented timeline, but the fledgling emergence of Two-Face (and yes, Harvey Dent's facial blemishes are there, if one looks beyond the explosive haze).
The theoretical, one-Joker-spawns-another stunt, though controversial, shouldn't be considered original or at all radical at this stage. Its roots exist in the Fox series, Gotham.
In the series, we're treated to a sibling combo for the Clown Prince of Crime: Cameron Monaghan's Jerome and Jeremiah Valeska.
Monaghan's brothers never played off each other in a blatant way, though both became facially deformed (in the Ledger vein), and in keeping with Joker tradition, they were both crazed: Clown Prince bookends, even if their personalities varied some. As it unfurled, one Joker fell to the wayside after a long-haul tease, allowing the next to seize prominence in a huge, aha moment.
So, there you have my humble, comparable stance. Again, I don't consider Foile a Deux's reputed twist too jarring (too out of left field), even if I (like some others) remain reluctant to exalt it as an established part of the Joker's cross-sectional canon. Nevertheless, if lightning has, in fact, struck twice, perhaps the two-Joker deal should be taken with less protest, if only based on what parallel Earth (what part of the mythology) one visits.
