Sorry, but this makes no sense to me at all. Why would they show us two of the Sith Lords when in fact they end up being the same person? Sure, they might want to confuse us, but from what the concept art shows, the two look like completely different characters. To me, Sion is someone who isn't afraid to end a conflict physically while the Masked Sith looks like someone who can gain just as much or more in the same conflict through other means like intimidation. Just looking at the concept art you can see that he (or maybe she) is a dominating presence.