CX will issue you a different boarding pass for the BKK -SIN leg even though you are on the same flight but as it might be 2 different booking classes it will beon 2 seperate PNRs.

As the transit time is very short, CX will arrange a ground personnel to meet you at the aircraft upon arrival and just hand you your boarding pass, stub off and show you your seat. (If its still the same one)

I've done this 'sector' a few times.