I’ve never liked “Obi-Wan had a second apprentice, one named Vader the other Anakin, both turn evil, and then one dies while the other secretly survives.
It feels awfully contrived, and distorts the meaning of the PT to benefit the plot twist in Empire. The PT is all about Anakin’s turn to the dark side as a parallel to Luke’s resistance to the dark side, and both are symbols of the galaxy - in the PT, the Republic falls to the Empire, in the OT, the Rebels resist the Empire.
Removing Anakin’s proper turn to evil is (imo) the equivalent of removing Luke standing up to the Emperor. Instead, we’d just show Vader rising and overthrowing the Emperor, leaving Luke’s story in limbo, until a plot twist in Episode 8 reveals Luke did the right thing. It doesn’t work. Likewise, adding a second apprentice even if to support Obi-Wan’s lie would be the equivalent of adding in a new character (Luke’s sister?) for the sole purpose of having us wonder who will redeem Vader.
Now, this is all implying Lucas’ vision of the PT. It could have been set from an entirely new character’s adventures / struggles and not Anakin, in which case maybe he could have showed up in the first film and then been seemingly killed. But as long as the trilogy is about Anakin’s fall or Obi-Wan’s failure, I don’t think you can thematically justify keeping the plot twist.
Well first off, neither one would be named Vader until after we see the suit. The second pupil character can be a natural part of the story from TPM, say Obi-Wan’s apprentice/partner before Anakin shows up. You already have the seeds of a rivalry and resentment between Anakin and the second apprentice, which can both fuel their turn. It wouldn’t just aid the mystery, but it would also add a bit of a familial dynamic to the main characters (Obi-Wan, Anakin, and the other apprentice; with Padme to the side). That would make the fall all the more tragic and kind of flip the OT’s themes (instead of the “rebel family” coming together, it’s the “Jedi family” falling apart).
I don’t think it’s contrived at all. We can see both apprentices fall in a way that satisfies the parallel to Luke standing up to the Emperor. You can imply that they both want to serve Sidious, without actually showing them bow to Sidious and receive the name.
You do bring up a good point about the “Obi-Wan’s failure” theme, as he should have been the main character of the Prequels. We see through him how he tried to train these pupils and failed, fuelling the fall of the Jedi and his exile to Tatooine to care for the offspring of Anakin (the other pupil would have been childless). It fits within GL’s general structure of the Prequels to center on Kenobi and you can work the story in a way that preserves the ESB twist.