The mind seems to be an emergent property of the various electrical and chemical processes in the brain - no one knows even how singletons work yet. Let's consider a mind to be a pattern within the brain processes. If you've got more than one of them in any given brain, there'll likely be times when one or two are ascendant (most active, whatever), and times when the active patterns mesh so closely that it's difficult or impossible to separate one from the others.

Is that the sort of theory you were looking for?