My ultimate suggestion would be not to force players to play close-proximity multiplayer. You're not just discouraging your fan-base, but forcefully limiting it.
In my opinion, it would be a better test and learning experience from both a designer's and a programmer's perspective to create a game that could use multiplayer, or an AI controlled team member. For instance: Army of Two.
Perhaps it would be even more creative to have the co-op campaign be DIFFERENT from the Single-player. The co-op missions could increase in difficulty beyond something a simple AI would be capable of?
Just a few random thoughts on the idea.