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