I've played Alien Shooter 2 co op campaign mode with a friend a long time ago, yet now, I can't seem to get it to work or find anything about it, yet I have vivid memoried from playing through the entire campaign, over hamachi I believe.
You likely won't see this post, but the game has had two versions. One named Alien Shooter: Vengeance and one named Alien Shooter 2: Reloaded. The latter has less music, less skills to worry about(Each weapon type had its own skill, and Armor had its own skill as well), stats were altered to an extent, a couple of survival modes added, and two levels added.
Oh, and they removed multiplayer.