This has been discussed a lot on the AGS forums too, and I think I'll restate my point: Al Lowe (who created the Larry Laffer series) spent several years with a team of the best pro adventure developers trying to figure out how one could make a multiplayer adventure game, but they finally had to drop the project after realising it couldn't be done. There were too many problems that couldn't be solved, like:
* player A picks up an object that is vital to the overall plot, and then logs off, never to return.
* player A picks up a vital object but doesn't realise it.
* player A and player B each picks up one of two objects that need to be combined
Ultimately, the problems could all be solved, sure, but the game would no longer be an adventure game - it would be an RPG.
You can read the full story at allowe.com, he firmly believes it can't be done, and honestly - who are we to argue with that kind of experience?
Scarpia