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Can Postal 2 and expansions be played Coop with friends?

It wasn't designed for it. That said, lots of games weren't exactly designed for coop, but you can still play it anyway. Like DOOM, which wasn't designed for coop but you can still use the multiplayer mode to play through the original levels with monsters turned on and experience the game in a coop way. Although some of the weapon placements aren't correct.

I believe there's either a way to make the game play coop via multiplayer, or a mod that does it for you. I remember reading about it, but I haven't looked into it myself. I'd use google.