Just figured out how to play a co-op game. you can have multiple characters in the same world. I have 2 guys at the same shelter (different game). However, items dropped in each game show up in the new one. This opens up the possibly (ok exploit) for co-op games for people to share worlds. Each person with their own characters.
you can zip up the cataclysm game and pass it around. everyone has their own character. however, you can pool items and take turns clearing areas.
There was a server with something similar, where while you couldn’t interact with other players directly, the server would save your map area once you left it and leave it for other to play with. If two players were in the same area, the game would just keep whoever’s left last.
But Cataclysm’s design would make it terrible for real co-op. Your game would get stuck each time someone is looking for something in it’s inventory or something.