In a recent post discussing the nature of individuality, I mentioned the paucity of examples of combined nervous systems or brains. Single brains that have been divided exist, but as I said, there are no good medical reasons to combine brains.
Well, I sometimes forget that nature runs little experiments of its own sometimes. Case in point: Krista and Tatiana Hogan. These two four-year-old girls, joined at the head, seem to have neural pathways connecting their brains, and while they have not been rigorously tested (due mostly to their age), they appear to be able to share some level of sensation, and occasionally they refer to themselves as if they are one. Their brain structure is unique, probably in all of human history.
So here we have a case, produced by nature, of two people who may be one person, or may be somewhere in between.