Rather than having two Vaders, why not play up the dual-persona angle, without revealing that Anakin is Vader? In other words, keep Vader an off-screen villain who does terrible things, until Obi-Wan faces him and says "YOU are Vader?!?" Vader puts Obi in a Force-choke. Obi-wan tries calling out to Anakin, who last we've seen is on the same planet. Vader says "...is no more"
I think that would keep the suspense without all this multi-Vader silliness...