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?
(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