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