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Due to *cough* hmrnrnrmrn rmrmr *cough*, some humans are born with two heads. This has been happening for at least as long as humans have been making cave paintings and it seems to be nothing to worry about (unless you don't get along with your bodymate, in which case you should contact a trained psychologist and refrain from fighting or biting yourself until an appointment is available).

They have two heads (no connection between the brains), a Y-shaped oesophagus and wind-pipe, a slightly larger chest to contain their marginally larger heart, lungs, veins and arteries to handle their increased oxygen requirements and (anecdotally at least) a tremendous appetite. However, as they could not be proven to be anything other than than human, further testing and experimentation hasn't been legally or ethically possible.

What other, if any, biological differences would two-headed humans show in order to operate just as efficiently and easily as single-headed humans?

--EDIT--
(From comment by OP)
@MontyWild Most of them can't tell because they generally don't disagree and coordinating movement becomes subconscious, but those that do fight claim they are both in control and that being shouted at and punched by yourself is too distracting to concentrate on which head is doing what

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    $\begingroup$ Is one head in control of the majority of the body, both heads in control of the majority of the body, or each head in control of its half of the body? $\endgroup$ – Monty Wild Dec 18 '15 at 0:49
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    $\begingroup$ @MontyWild Most of them can't tell because they generally don't disagree and coordinating movement becomes subconscious, but those that do fight claim they are both in control and that being shouted at and punched by yourself is too distracting to concentrate on which head is doing what $\endgroup$ – MrLore Dec 18 '15 at 1:22
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    $\begingroup$ Have you researched how the real two headed humans deal with this? Siamese twins are a well documented phenomena. There's even curious ones which, unlike your particular 2 headed human, share a brain connection. Fascinating topic. $\endgroup$ – Cort Ammon - Reinstate Monica Dec 18 '15 at 2:28
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    $\begingroup$ Generally speaking, two-headed humanoids become President of the Galaxy specifically to ensure they can steal a starship with an infinite improbability drive, and invent the most devastatingly alcoholic drink known to man. That's pretty different, I'd say. $\endgroup$ – Spratty Dec 18 '15 at 8:47
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    $\begingroup$ I'm voting to close this question as off-topic because it shows a lack of effort or research on the author's part. $\endgroup$ – fi12 Feb 20 '16 at 12:46
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There is a real-life case of almost-exactly what you are asking about. Abigail and Brittany Hensel are a pair of dicephalic parapagus conjoined twins. They each have an individual head, but share a single body beneath, though there is some duplication of organs above the waist (for example, two hearts and three lungs). They each control and feel one side of their conjoined body, but because they grew up that way, they are very coordinated -- able to ride a bike, drive a car, and play piano, for example.

At least in the real world (and this may be different in your scenario), conjoinment is an abnormal condition -- i.e. it's not a programmed genetic trait -- so the biological specifics are randomly unique to each case. As a result, most cases end up with various health conditions, and many don't survive infanthood. So far, for the Hensels, this doesn't seem to be an issue, as they are doing quite well. There is quite a lot of information about them on line, but here is a decent article that gives a summary, and some additional details: http://www.buzzfeed.com/moonc/everything-about-conjoined-twins-abby-and-brittan-a1kb

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  • $\begingroup$ The Bunker brothers. The original Siamese twins... en.wikipedia.org/wiki/Chang_and_Eng_Bunker $\endgroup$ – Smoj Dec 18 '15 at 14:53
  • $\begingroup$ Yes, advanced medicine NOW can enable them to be born, I assume C-section. In natural conditions, such pregnancy would kill the mother. Such trait cannot "evolve" without significant changes to birthing canal. $\endgroup$ – Peter M. - stands for Monica Feb 17 '16 at 23:25
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If there is no connection between the brains, then one of them is probably useless to the body. That also means that this useless head cannot control the body.

If the two heads can control the body somehow, they can sleep in different times, keeping the body active all the time, which would be extremely necessary since eating more than usual humans means they will also have to gather more food. If the two brains studied and learned different things from one another, they can complement each others knowledge to a more intelligent being.

In both cases the other head serves as an additional opinion to ask for anytime you need.

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One big problem would be how such humans are born.

Major problem and major limit for human development is the size of the head and how it can pass through birthing canal.

I assume that Hensel twins were born by C-section. Which is fine in developed world, but not so much in natural habitat in stone age.

So main problem in your world is not how such humans will live, but how they can be born. Without advanced medical care, they cannot be born, and unresolved pregnancy would kill the mother. So such trait cannot "evolve" naturally - any gene increasing chance of two heads would be eliminated by darwinian pressure.

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As I remember, there is one other case of a two headed adult. The story is that in early 17th century France (roughly time of the Three Musketeers) A two headed sideshow freak killed a man. He was not executed because they didn't know which head was guilty and they didn't want to execute an innocent person.

I doubt that they would have been born by C section that early, so presumably they survived a natural birth.

Added October 15, 2019.

Here is another case of adult two headed humans. Giacomo and Givovanni Battista Tocci were born between 1875 and 1877 and retired from their career as a circus freak in 1897; reports differ on how long they lived afterwards.

https://en.wikipedia.org/wiki/Giacomo_and_Giovanni_Battista_Tocci

Wikipedia lists 11 examples of two headed humans born between 2000 and 2014 and doesn't say that all of them died, so some of them might also still be alive.

https://en.wikipedia.org/wiki/Polycephaly#List_of_recent_occurrences

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  • $\begingroup$ C section was know to the romans (no Cesar was not born that way) probably learned from the greek but it involved the mother dying. But without a c section both mother and child would die anyway. $\endgroup$ – BentNielsen Aug 27 '16 at 17:30

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