I find it wild that the body can still function in those rare cases. Like I assume that there’s a great deal of stress on the heart and stomach etc since they have to support two “minds”? When the conjoined twins eat, do the two sources of food intake end up in the same stomach? I also know a case where the two shared a same brain, and they knew each other’s thoughts.
Also, I’m curious if even more extreme cases of conjoined triplets are possible or have happened.
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It depends on how they are conjoined. There are some types where parts of the digestive system are shared, some where they aren’t. Some will have two hearts, some a shared heart.
No one is quite sure how they happen, but there are theories.
The simplest answer for how the body works is that for the most part it doesn’t. Most conjoined twins are stillborn or die shortly after birth.
Generally the more they share the less likely they are to survive, but as with all of biology there are exceptions.
Triplets are possibly but extremely rare (confirmed cases in the single digits).
The [Wikipedia page](https://en.wikipedia.org/wiki/Conjoined_twins) is probably a good place to start further reading, particularly the “Types” section, and the “Notable people” one.
In many cases they do not survive. The ones who make it are the exception. The fewer tissues are shared, the better the chance of survival (and the more likely it is that they can be surgically separated).
As for how they happen: the most likely theory is that conjoined twins are the result of the same process that results in identical twins, i.e. a single fertilized egg cell splits during development but not completely.
Your other questions depend on the specific case. Some conjoined twins share a heart and/or bloodstream – others don’t. Some have one stomach each, while others have just one between them. And so on.
As for “sharing the same brain”, there is a case *like* that but you’re exaggerating it a bit (and I don’t blame you because many news reports have done the same thing). I assume you’re referring to the [Hogan twins](https://en.wikipedia.org/wiki/Krista_and_Tatiana_Hogan), and they definitely each have their own brain. However, there is evidence that their nervous systems are connected. How much so is not quite clear as of yet (at least from the sources that I could find). It seems that each twin can see out of the other’s eyes, to some extent, and can feel when the other person’s body is being touched. But that’s not quite the same as knowing each other’s thoughts. Further research will be necessary to determine how extensive and profound the connection is, but for my money (with the limited expertise I have in this as a neuroscientist) I think that sharing sensory inputs is a lot more plausible than sharing thoughts.
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