Heres the scenario:
Hannibal Lecter is out of police custody and is hiding in Gotham. He finds out about the joker and thinks it might be interesting to eat him, and perhaps study his psyche. The joker also learns of the man and for whatever reason, decides it would be hilarious if he killed him in some qrotesque manner.
So basically, neither know where the other is, life in Gotham continues as normal but they're both trying to kill each other. GO.