It has two intertwined campaigns with different protagonists.
Gamereactor recently got to check out Alan Wake 2 after it was shown during Summer Game Fest last week, something you can read more about it in our preview. One of the main new things in this adventure is that our main man Wake has to share the spotlight with FBI agent Saga Anderson.
The both have stories of their own (which means you cannot play Alan Wake in a chapter made for Anderson, and vice versa) and you are free to choose in which order you do their respective missions, save from the very beginning and towards the ending. In a recent Q&A, Remedy answered several questions about the game, and one of them was about the length of this adventure.
Communications director Thomas Puha had this to say on this topic:
"It depends on your skills, but currently (and the game is not quite done yet because a lot of the content is still going in), we're looking at over 20 hours."
This is quite massive for a horror game, but hopefully, Remedy has a story well written enough to keep us entertained all the way through.