It's hardly possible, becouse there is a wallpaper (concept art) where this Nihilus dude and Atris are fighting. They can't be on the same scene on the same time if they are the same person.
But what if the identity of the masked guy is depending on what choises did you make on the begining of the game:-) He could be Bastila, but he could also be Revan:-)
Well. As concept art, it could represent an internal struggle or perhaps just represent a light versus dark side confrontation. I wouldn't read concept art as representative of what will really happen.