I assume the OP is doing HKG>>>SIN>>>CMB>>>SIN. Assuming it is on the same metal, then it will be CX 711, then returning as CX710 exCMB.

Assuming that this is a legal booking and ticketed, the check-in agent in HKG should be able to tag the baggage to SIN with the 2 separate flight number.

Better to check with CX itself. I don't think anyone for that matter has done something similar before.