The co-DM's are also players. Regarding feelings towards being exposed to the plot, I think that the normal DM has devised a way that the co-DM's get only a limited amount of knowledge. He might pass out the specific goals of the night's adventuring, but not reveal much more than that. He sometimes sets up lists of items that we need to acquire from around the world, gives one of those to a co-DM, and tells them that they can fill in the rest of the campaign with their own encounters and settings. (I imagine that it's easier at this point since we have access to an entire world, and can sprawl as far as we want from our home base!)
The DM usually employs one or two people to do the majority of his two-sided campaigns (usually one of his closer friends or his girlfriend) so I think that helps with limiting how much of the plot is revealed. Picking someone who doesn't mind spoilers and who still enjoys playing for the sake of seeing things unfold would be the best choice for a co-DM, in that case!
The DM usually employs one or two people to do the majority of his two-sided campaigns (usually one of his closer friends or his girlfriend) so I think that helps with limiting how much of the plot is revealed. Picking someone who doesn't mind spoilers and who still enjoys playing for the sake of seeing things unfold would be the best choice for a co-DM, in that case!
