This fascinating New York Times Magazine articles tells the story of conjoined twins Krista and Tatiana, who share part of their brains; specifically, there is a bridge of neural tissue joining their thalami. The thalamus is something like a switchboard for routing sensory information.
While the twins have two distinct minds and personalities, each can see and feel the other's sensations. It's an interesting natural experiment in personal identity:
‘I have two pieces of paper,” Krista announced. The girls sat at a small table in the living room, drawing, their faces, as always, angled away from each other. Each had one piece of paper. So I was surprised by Krista’s certainty: She had two pieces of paper? “Yeah,” the girls affirmed in their frequent singsong unison, nodding together. It was one of those moments that a neurologist or psychologist or any curious observer could spend hours contemplating. Was Krista using “I” to refer to both her and her sister? Is Tatiana agreeing with her sister’s assessment at a cognitive level or uttering the same word simultaneously for reasons unknown to her?
In addition to sorting out the usual sensory experiences of the world, the girls’ brains, their doctors believe, have been forced to adapt to sensations originating with the organs and body parts of someone else.
The article hints at the question of what qualitative difference there is between, say, Krista's own sensations and those she gets from her twin; for instance, clearly she is seeing what she sees and what Tatiana sees at the same time in some sense without getting confused. But it's unclear exactly what "seeing" means here; whether she's simply reproducing Tatiana's experience (and if so how she coordinates that with her own experiences, how visual fields are related, etc.) or whether she's engaged in some sort of hyper-empathy (or another sort of less direct experience). As the author puts it: "If the two girls are unique individuals, then each girl’s experience of that stimulus would inevitably be different; they would be having a parallel experience, but not one they experienced in some kind of commingling of consciousness." Is the following some evidence for the latter?: "... I absent-mindedly gave Tatiana’s foot, which Krista could not see, a little tickle. She turned to me and smiled, and then Krista spoke: 'Now do me,' she said." The author points to another possibility: "Had she felt the sensation but wanted the emotional experience of knowing that she, too, was receiving that kind of playful attention?"
And then, sharing sensory experiences does not mean liking the same things:
The sensory exchange, they believe, extends to the girls’ taste buds: Krista likes ketchup, and Tatiana does not, something the family discovered when Tatiana tried to scrape the condiment off her own tongue, even when she was not eating it.
And fighting gets compicated:
This time there was no fighting over the two different sweatshirts. On the rare occasions when the girls do fight, it’s painful to watch: they reach their fingers into each other’s mouths and eyes, scratching, slapping, hands simultaneously flying to their own cheeks to soothe the pain.
The girls were tired. It was late for them. Someone ordered them chicken fingers, and Krista took a bite. Suddenly, Tatiana made a face. “It’s too yucky,” she said, starting to cry. The mayhem level went up a notch, and Tatiana crawled under the table, wailing, as Krista was trying to pull her back up by the force of her neck. Krista tried to put the chicken finger directly into Tatiana’s mouth. “Krista likes it!” she said. “It’s yummy!” Tatiana spit the food out, crying: “Let me hide! Let me hide! ... “I am getting out of here!” Tatiana sobbed. “Let me alone.”
And finally on the question of identity:
Although each girl often used “I” when she spoke, I never heard either say “we,” for all their collaboration. ... It’s like they are one and two people at the same time,” said Feinberg, the professor of psychiatry and neurology at Albert Einstein College of Medicine. What pronoun captures that?
The author includes an inevitable shout-out to a popularizer of philosophy of mind/cognitive neuroscience (Damasio), and an equally inevitable reference to "mirror neurons" (because apparently the word "empathy" no longer deserves our sympathy).