How do you know? How could you possibly know?
I don't dispute that there is not a one-to-one correspondence between consciousnesses and bodies. Multiple consciousnesses could inhabit the same body (multiple personality, split brain, conjoined twins) and a single consciousness could be distributed across multiple bodies (I don't know of any examples but I can't think of any reason this should be impossible in principle). But there's a difference between receiving input from someone else's sensors and experiencing their qualia. It's the difference between, "This ice cream tastes like pistachio", and "This ice cream (which tastes like pistachio) tastes good." To demonstrate someone experiencing someone else's qualia you'd have to find an example where the two individuals had different preferences about something (like pistachio ice cream) and having one of them simultaneously experience liking it and not liking it. I don't see how that could be possible even in principle.
> there's a difference between receiving input from someone else's sensors and experiencing their qualia.
There can not be clear difference if you are not able to even fully separate two different personalities.
If distinction between personalities is fluid, then distinction between personalities qualia also has to be fluid.
> If distinction between personalities is fluid
But it isn't. It might be vague but it isn't fluid. If two personalities are distinct then they remain distinct. It might be difficult to decide whether two different kinds of behavior are manifestations of two different personalities, or the same personality behaving differently from one moment to the next. But whatever criterion you apply, the situation is not going to change from one moment to the next. This is one of the defining characteristics of personalities. It is what allows you to say that a person is the same person as they were yesterday, despite their behavior today being different in some respects from what it was yesterday.
Additionally, the process of separating parts of a brain is also continuous in time. People could be experiencing qualia when then brain is being damaged and the process could take long enough for them to register the states in between.
With regard to reports, here:
> He also reported to have grabbed his wife with his left hand and shaken her violently, at which point his right hand came to her aid and grabbed the aggressive left hand.
And here: https://youtu.be/N1Mac4FeKXg?t=124
With regard to definition of personalities - definitions created for an average case are often not sufficient to describe the reality. Quantum physics is a perfect example how reality can break old definitions.
I see that as evidence of two consciousnesses/personalities sharing one body (i.e. one set of I/O devices), not two personalities sharing qualia.
When you go to a large event such as a concert, there is an "atmosphere". Do you think this could be considered a collective consciousness? You might dismiss it straight away, but this would be like a single neuron assuming that there isn't a higher level of consciousness built on top of it (assuming that the neuron is responsible for part of the consciousness).
> Though Krista and Tatiana Hogan share a brain, the two girls showed distinct personalities and behavior. One example was when Krista started drinking her juice Tatiana felt it physically going through her body. In any other set of twins the natural conclusion about the two events would be that Krista's drinking and Tatiana's reaction would be coincidental. But because Krista and Tatiana are connected at their heads, whatever the girls do they do it together.
I also recall examples where one girl does not like eating something and so the other girl can not eat it because the other can feel it.
Concept of an individual is not a fundamental property of the universe. It is an emergent, fluid and complex concept.
There are people with multiple personality disorders. There are brain conjoined twins. There are people with severed corpus callosum.
E.g. here is report about one person [1]:
> After the right and left brain are separated, each hemisphere will have its own separate perception, concepts, and impulses to act. Having two "brains" in one body can create some interesting dilemmas. When one split-brain patient dressed himself, he sometimes pulled his pants up with one hand (that side of his brain wanted to get dressed) and down with the other (this side did not). He also reported to have grabbed his wife with his left hand and shaken her violently, at which point his right hand came to her aid and grabbed the aggressive left hand. However, such conflicts are very rare. If a conflict arises, one hemisphere usually overrides the other.
[0] https://en.wikipedia.org/wiki/Craniopagus_twins#Media [1] https://en.wikipedia.org/wiki/Split-brain