rpvee said:
It's also not like Sith/Dark Siders haven't had two names before in Star Wars history. He could still be Plagueis.
If they do go that route, they better have the sense to at least refrain from calling him by that name. Identify him as the master of Palpatine if they have to, but don't refer to him as Darth Plagueis, Plagueis, or any variation thereof.