More accurately, two girls sharing one body:
More info, including a map of the complex layout of their internal organs, can be found here
The amazing thing about these girls is how healthy and functional they are. Although either girl can control their shared arms and legs, and can "fight" over the control if they choose, they can coordinate well enough to ride a bike, and even play baseball well. And one girl can choose to "hand" control over to the other. It's incredible.