Pityesz: Exactly as the changelog stated (at least the official one). Sadly it need two controllers and will not work with keyboard + mouse and controller. Then have to start the game with the "-coop" parameter. When in the game just press start on the secondary controller.
The coop is very oldschool in this. Both players move on a single screen making it some tight situations as chaotic as hell and everything is shared between the two players. If you finished the bonus dungeon you have to make sure at least two of the characters are active. We haven't tried remapping the second controller I don't know if it is possible. AI controller team members receive half damage but if a second player take control of it that defense is removed from the given character. Interesting things may happen like tossing around each other or whatever physics based madness. XD
Yeah I've read about the 2 gamepad requirements, I wonder if this would work through parsec, because if so, this could be truly interesting.
As far the -coop parameter, durante said that for GOG version you just have to run the game directly from the exe, without mentioning the parameter at all, which is kinda odd (but better, since I don't need to mess with parameters).
Btw thanks for the detailed info!