Well, Ninja Theory had originally planned to include an online deul mode for Heavenly Sword (which was then supposed to come as a free update after the game released :/ ), so maybe it's something like that.
I don't think it'll be co-op, as that would involve two Kratoses running around, which would just be wierd. If it's a deul mode, however, they could then include any other hominid bosses from the game.