I played Grim Fandango with my girlfriend - playing adventure games cooperative is better than watching movies, if you got a little patience.
You're right. When I was 10-15, I played the SCUMM games with my school friends. Sometimes we gathered around the same computer to play for a while, but not always. We also played alone, and when we met during school, we talk about what we were able to do on our own last evening. And together we were always able to finish the games.
By the way, before Internet and those too-easy-to-access walkthrough, adventure games were interesting multiplayer games, even though the "multiplayer" functionality was not directly integrated to the game. Unfortunately with the walkthroughs, people tend to get lazy so this doesn't quite work anymore. But I'll tell you (though, if you are true fans of adventure games, you probably know it...) Walkthroughs are EVIL... [>