Those whose corpus callosum is surgically severed are said to be patients with

split brains

Explanation:

The corpus callosum is a white matter tract of nerve fibers that connects both cerebral hemispheres, facilitating interhemispheric connectivity.  The role of the corpus callosum in epilepsy is the interhemispheric transmission of epileptiform seizures that are generally synchronous bilaterally.

A corpus callosotomy is an operation that severs (cuts) the corpus callosum, interrupting the spread of seizures from hemisphere to hemisphere. In addition to interrupting this synchrony, the callosotomy decreases the frequency and amplitude of the epileptiform seizures.

Those whose corpus callosum is surgically severed are said to be patients with split brains.