The human brain has two HALVES or hemispheres. While in most humans the hemispheres have separate functions but operate together, there are rare cases in which the connections between the hemispheres dont develope or become severed. In those cases those people do essentially have two separate brains and it can cause a lot of functional problems. There are even rarer cases in which half a persons brain does not develope or a person has half their brain removed due to illness or injury. Interestingly in these people the remaining brain can learn to take on the functions of a whole brain and the person can live a more or less normal life.