Just in case you were talking about a 'quad hemispheric' brain design.... There might be advantages in terms of redundancy. Humans would normally have diminished capacity from damage to one brain. Perhaps dual brained humans would be resistant unless identical damage was done to both. Humans would not need to sleep if both brains alternated doing the task of facilitating the person's consciousness. Humans could fully focus on more that one thing at the same time. Disadvantage -- possible problems caused by increase need for blood flow (needs larger stronger heart). Increased oxygen intake.... will need larger lungs Radical changes required in "skull capacity" as well as changes to supportive muscular and skeletal attributes (bigger thicker neck). You may need to re-route eyeballs and connecting tissues.