[drited]

The people who have had half their brains removed (Hemispherectomy) and are functional had it removed when they were very young and the other side wired to adapt. Depending on age of operation, there may be more or less significant impairments for life.

For humans with 2 healthy hemispheres, as far as I'm aware better imaging is revealing that split brain theory is only partially correct for many of the processes that were formerly thought to happen in one half only. Recent imaging has shown that the other half is still involved, albeit in a less widespread fashion.