This question is inspired by Westworld. Spoilers for it and for other shows/movies are inevitable, so be forewarned.
In Westworld, the prevailing theory is that William is the Man in Black, just 30 years ago. I’m confident this should be a trope, but I can’t find an entry for it. Circumstances:
[ul]
[li]The same character is played by two (or more, I guess) actors.[/li][li]It’s done for the purpose of disguising (from the audience) that they are the same characters.[/li][li]It’s not done just because they are different ages (like casting a child to play a young Hero) unless the audience is specifically deceived in doing so.[/li][li]It’s not done just to replace an actor who left. (The two Darrens, or James Bond)[/li][li]It’s not done across several works (Ewan McGregor versus Alec Guinness as Obi-Wan)[/li][/ul]
I’m positive I’ve seen other examples of movies/shows where this has happened, but I’ll be damned if I can think of them. The closest I can get is Fight Club, but that’s not quite it.
Not exactly the same thing, since it wasn’t two actors playing the same character, but I’m reminded of Natalie Portman and Keira Knightley in “The Phantom Menace”, in which both of them were in the role of / costume of Queen Amidala. However, when Knightley’s character, Sabé, was in the “role”, she was acting as the queen’s body double / decoy (and her lines while in that role were overdubbed by Portman, to add to the “disguise” aspect).
Looper? It’s been a while since I’ve seen it. Can’t quite remember if we were supposed to know who was who/when.
Fight Club is the obvious example, where even the character himself doesn’t realize it.
How about Guy Pearce and Stephen Tobolowsky in “Memento”?
Predestination, the movie adaptation of Heinlein’s “All You Zombies”.
12 Monkeys has the child and (time-traveling) adult version of the same man seeing each other without either realizing the other’s identity.
Luis Bunuel’s That Obscure Object of Desire has the same female character played by two different actresses because each represents a different state of mind of the lead male character (though this isn’t obvious at first).
The Watcher in the serial “Logopolis.”