I did this several weeks ago. Checked in online for the CMB-SIN CX710, but no self-printed boarding pass available. While checking in at SIN for CX711 SIN-CMB, I asked if they could send a message to the CMB station to tell them I would be checking in for the return flight at the UL transit desk at CMB. Got off the plane at CMB and there was a UL ground staff with my name on a board at the gate, and she handed me my boarding passes for CMB-SIN-HKG that day and HKG-LHR the next day, so I was in the lounge 3 minutes after disembarking with an hour and a half to kill before the return flight.