In a well written story I can see that working. In fact, I'm totally willing to buy that Batman has a blindspot when it comes to the Joker in general. I can see him totally buying into the unpredictable chaos aspects of the Joker, when Dick or Tim could look at something Joker was doing and just go "Oh, those are just some old popculture references he's basing his crimes on. I bet next he'll ..."
Sadly though, this wasn't a smart story.
Even given the idea that he COULD create them, if there are two other completely unpredictable spotlight hogs out there in addition to the real Joker, how have they never gotten in each other's way over the years?
Has there never been an instance where Joker robbed the first bank of Gotham while at the same time across town was kidnapping famous comedians?
Has there never been two Jokers show up at the same time trying to recruit a gang for their latest schemes?
Has Batman never tracked down Joker's lair only for it not to be the Joker that did the crimes he's investigating?
If somebody writers a bad article about Joker in the paper, why does only one Joker show up?