I think this happened a whole lot in Marvel Cinematic Universe, especially when the Guardians of the Galaxy characters met the various Avengers, and the main one when Tony Stark met Dr. Strange (who is basically just magic Tony Stark). There was lots of fan hype for that happening.
I think it's when there are characters from completely different "subplots" in a story that meet for the first time, is when fans get really excited, especially characters that are quite similar to each other in specific ways but are generally different otherwise. You saw that happen a lot in the comic Homestuck when the trolls started talking to the human kids more.
Also, in my book Hands Held in the Snow, any reader knows that the first time Tia and Runa finally meet... It's an important moment, let's just say that...