What is the treatment and prognosis for someone with a combination of carotid occlusion/stenosis and occlusion/stenosis of the basilar artery?

Depends. Artery disease in the neck vessels (carotid/basilar) is associated with risk for stroke. Your doc, vascular provider can best assess all the associated factors and give you a risk assessment. Treatments vary from medical management, nicotine cessation, stenting, surgery and combinations of them all.
Surgery. The brain gets its blood supply from 2 carotid arteries and 2 vertebral arteries that combine to form the basilar artery. These 4 vessels are the only sources. The vertebrals and basilar are generally not approachable by surgery so the carotids are often operated to improve circulation if there is >70% stenosis. Antiplatelet drugs are also used.