Ziz said:
Ok, I didn't think of it that way. I figured the Joker was so confident that it wouldn't matter which one Batman or the cops went for.

Where was it mentioned that Joker swapped the addresses?

As others have stated, it's more implied than explicitly stated. The closest they come to doing so is when Batman and Gordon are mobilizing to save the two. Gordon asks Batman who he's going after. He says Rachel. Gordon tells his men that they're going after Dent. So when Batman opens the door and sees Dent, you're supposed to go, "Oh, shit. The Joker switched addresses on him."