We do have a natural experiment which may provide some insight into consciousness: conjoined twins. They seem to be able to both think independently but also communicate wordlessly.
This applies to any two humans, not just twins. Ever smiled at someone and had them smile back? Ever waved at someone and they waved back? Ever shook your head at someone's behavior? We communicate wordlessly all the time.
If one twin had direct neural access to the other twin's emotional states or "what it's like" aspect of sensory perception, that would be different than any form of communication involving one making gestures that the other interprets.
I don't believe we have any reliable evidence for that though.
No; the cases of conjoined-at-the-brain twins where both twins are conscious to the degree of each knowing that it is a person are extremely rare, and in any case, a dualist could always claim that whatever is physically observable is not the whole story, as dualism is unfalsifiable so long as its proponents avoid making any affirmative statements about how the mind works.
They have independent brains though. I don’t think it’s a useful data point.