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