During The Dark Knight, I can't help but think of a very strange- yet, very cool hypothesis...
It is during this scene...
...where something she says sticks out.
Rachel is speaking to Harvey and she says, "They're going to let one of our friends decide." As in choose which one of them is saved. Remember that scene? Of course you do.
But doesn't that sound a bit odd? That one of The Joker's mercenaries specifically says that one of their "friends" will choose? Key word: Friends.
There are two options as to who this friend could be: Jim or Bruce, and there are two moments in the film where someone explicitly states that she is friends with both.
1.) In the beginning of the film Rachel mentions to Harvey that Jim is a friend of hers, however she doesn't say it in front of anyone else but Harvey. So, no one can really know that Rachel and Jim are friends. There's no other internal logic in the film that can clarify otherwise (for every "flaw" people point out in the film, there is at least one piece of internal logic in the movie that clarifies it, like why Batman doesn't blame Harvey's death on The Joker). Since it is unlikely that The Joker hears this, I do not think it's the connection between Rachel and Jim that The Joker (and his henchmen) refer to.
2.) I propose that in the fundraising/party scene, the one where The Joker crashes the party, that The Joker makes (or had previously made) the connection between Rachel, Bruce, and Harvey.
Think of it this way: The Joker (who comes up with the most intricate, yet somewhat plausible, evil scheme ever) can ask (and probably does ask), "Where's Bruce Wayne, the young, muscular guy who was gone for years and then came back right when the first sighting of "The Batman" occurred?" Well, he has just finished beating the living shit out of one of his henchmen here:
...but no one saw that... Oh, Bruce isn't here, but it just happens to be his party?Now that doesn't make sense.
Wait, what's that? It's Batman! What a weird coincidence! Not that The Joker doesn't expect Batman to show up, it's just that Batman shows up at Bruce Wayne's party, where Bruce says his oldest friend is Rachel, and where Batman "throws himself after" Rachel to save her life.
At this party, Bruce explicitly states that Rachel is his oldest friend. In my opinion, this situation is much more likely to have been overheard by The Joker or one of his henchmen than the one at the beginning (meaning the connection between Bruce and Rachel is established, rather than the connection between Rachel and Jim, along with the well known line that The Joker mocks Batman with: "The way you threw yourself after her, "kinda makes you think that The Joker knew who he was... right? No? Okay...)
I also feel like this adds to even more of a reason he doesn't want anyone else to know his identity at the end of the film.
The moment I'm referring to is...
He doesn't want Reese to spoil all the fun.
Well, I'm not sure how much more of an argument I can make and it's a pretty weak one, but I sure do like to think it could be true because if it was...
...wouldn't it change the way you see how The Joker looks at Batman?
Food for thought!