All that matters for the same identity to me is the brain. If one can read the thoughts of the other so easily, in my book it is the same person.
Disclaimer: am resident in Neurology with a background in Anatomy
I can also read your thoughts. Are we the same person? The only difference is you need to put them in text, and she does it via electrical impulses.
Their thoughts are not integrated into a single entity, but rather they are able to communicate via a channel you don't posses. As evidence, when they need to share thoughts it takes them extra time, to me that means communication, not single entity.
Another way to put it: They can distinguish their own thoughts from those of their sister.
> am resident in Neurology
Then you should know that even a regular brain is really multiple independent "thinkers" that are wired to give the illusion of a single entity. Disrupt that wiring and the different "minds" will do their own thing, separately from the others (alien hand syndrome for example).
And I know that that sounds like an argument against what I'm saying.
It's basically the question of consciousness: Who is the "I" that is speaking. That alien hand has no consciousness. These conjoined sisters have two, because each can perceive itself as distinct.
Also something to ponder.
And heck, as disturbing: what about two (regular) twins raised as one person...
They still seem to have ONE thalamus and that says a lot about their ability to perceive each other side.
Because the system isn't well-defined in this case, which happens less than 1 in a million times, so isn't worth trying to fix explicitly.
The simplest solution is to give them two identities and move on with life having a little oddity -- as a pragmatic solution, it seems like the one that would end in the least litigation, as it confers an advantage not a disadvantage on the person(s) in question and can't possibly disenfranchise anyone.
It's not like you can't already create extra 'selves' (in most senses; eg voting is different) legally at the cost of $200 a pop -- what does it really matter if less than 1 in a million people happens to have two for what is otherwise going to be a very hard and likely short life?
That said -- I agree with you that it's not really well-defined here, because our notion of 'identity' is already highly problematic.
Edit:
This is a strange thought: what is the impact on someone with multiple personalities if you allow each personality to incorporate an LLC, all owned by a parent LLC (obviously, all legally owned by the 'person') -- ie, create an external legal framework that parallels the structure of the internal mental one, that all of the personalities should be able to view?
I'm honestly not sure if that's helpful for providing a lens to see the interaction play out or harmful for legitimizing the split. (And to some degree, all of us possess such multiple personalities -- we're rarely entirely the same person at work as at home or church or... whatever. I'd just argue that 'healthy' adults understand the interaction better.)
As far as I'm concerned what matters is how many entities exist who display a behavior of a separate entity (at least to the degree that we acknowledge as being separate) - i.e display different personalities each with a cohesive structure and integrity. It really doesn't matter how many physical brains they share.
SPLIT was a fantastic movie in any case! :p
Schizophrenia has to do with some degree of failure to separate reality and delusion. It's got nothing to do with multiple personalities. Are you talking about dissosiative identity disorder, instead?