User blog comment:Master Sima Yi/Assassinews 12-05-'15 — Assassin's Creed: Syndicate has been revealed!/@comment-5174994-20150512182658/@comment-5174994-20150512183250

Damn, I should have read on. This quote from Ubi literally confirms it:

"I think two protagonists is something that can bring a lot of freshness to the way that we do storytelling. We tried that on Assassin’s Creed 3 with Haytham and Connor, but they were coming one after the other. So, one of the things I was wondering was, what if we had those two bouncing off each other. “They’re twins, with the playfulness of twins, but still have their own very different personalities. This was the intention we had from the beginning – to bring a different tone to the story.”