Spinks has a good post on Duoing in MMOs, where she examines content in MMOs and pairs of players. I'd just like to add another thought to this discussion.
One important point about duoing that’s not often considered is that it has a large impact on system requirements.
A couple or family rarely has two top-of-the-line computers. They might have one edge computer, but the other computer will be a few years older. Then the oldest computer gets an upgrade, and the other person lags behind. Or the second computer won't be a desktop machine, but a laptop geared more towards work and websurfing.
So it’s important that your game be playable on the *second* household computer. I think that is where WoW has a marked advantage. A lot of newer MMOs are definitely playable on the first system, but won’t run on the second system in the household, and so the couple cannot play together.
Where a single-player game can target the spec of the first computer and still count on a purchase, an MMO or co-op multiplayer game really needs to target the second system in order to be considered a viable option for many players.