There is only one Main Character in a story. Why is this? Because each complete story is a model of the Story Mind which reflects our own minds, and in our minds we can only be one person at a time. At any given moment, we have a position in our own thoughts. Our state of mind in regard to a particular problem reflects the biases of the position on which we stand. If a story is to fully involve an audience, it must reflect this point of view.

From the Dramatica Theory Book