Or...how about this: per Samuel Vimes Both Anakin and Darth go to the Dark Side. However, it is Vader, rather than Skywalker, who is killed. Anakin then secretly assumes the identity of Darth Vader, but Obi-Wan, Yoda, The Emperor, and pretty much everyone else, assumes that it was Darth who survived, and Anakin who was killed. I believe an issue of Fangora magazine from 1981 that stated as much; the author's alternate speculation was that Vader was merely a clone of Luke's father (and that the Emperor was a clone of Ben). That said, I believe that both your scenario and that of Samuel Vimes cohere with that obscure note from The Making of ESB more than any other alternate idea out there.