Not really, but to give Obi Wan a second apprentice would just mess the OT in a worse way. Because at the end of the day, we've seen Anakin's fall anyways, and we've seen obi wan fight him.
Even if all the signals pointed that Vader was this second character, we would have presenced Anakin not being as great as Obi Wan described him. We'd even know that ObiWan is lying because if Anakin dies at mustafar, it wasn't vader who killed him...
That would make the ESB moment be a meh, it's not X, it's the other bad guy turned evil in III.
PT set up a story which is far from this possibility with the material we've got.
Besides there's great potential in the story to make it a real tragedy, really.