At HKG, Singapore Airlines is in Terminal 1 and Bangkok Airways is in Terminal 2. There is an "Automated People Mover (APM)" that transports people between the terminals every 2-4 minutes, so that should not be a significant time concern. Since you are switching airlines for your connecting flight, you will need to go through immigration, customs, and baggage claim in Terminal 1, and then get to Terminal 2 departures to check in for your Bangkok airlines flight. If the lines at immigration and customs are unusually long, you may have an issue making your connecting flight. But under normal conditions, you should be able to make it to Terminal 2 within 60-90 minutes of landing at HKG.