Both destinations are served by Guwahati (GAU). Since there's no flight route between them, the best way to travel is by road.
Bongaigaon
Fly to Guwahati (GAU), then travel by road to Bongaigaon. Taxis, buses, and private cabs are available from the airport.
Guwahati
Fly to Borjhar (GAU), then travel by road to Guwahati. Local transport options include taxis, auto-rickshaws, and state buses.