A behavioral neurologist completed a neurology residency. A neuropsychiatrist completed a psychiatry residency. Then both of them went to the same fellowship.
In historical times, there was no difference between a neurologist and a psychiatrist. A non-surgical neuro clinician practiced neurology, psychiatry, and neuropathology. A famous example was Alois Alzheimer. The founder of neurology, Jean-Martin Charcot, was a pathologist, and went on train Sigmund Freud!