Is it me or can up to 4 players play at the same time when 4 characters are together in parts of the story? Unless Jake, Chris and Piers use random usernames but I doubt it. Chris was called Schneeloke, Piers Reploid99, Jake Godzyla75, and Sherry has no username, which is why I'm thinking it's other players that joined me even if the game didn't warn me about it. They also were acting differently than the CPU and I don't remember seeing Jake having a username the first time I got to this part of Chris's story.
Yes, it is possible that you hook up with 2 other players when you play a part of the story where both campaigns intersect such as Chris and Jake team up during those big B.O.W. I think its pretty cool :o