It is not possible to combine 2 PNRs on the "reservations" side of Amadeus.
However, when checking in at the counter in SIN, the agent can combine the "customer records" of the SIN-HKG and HKG-LHR flights into one by using the "Find Connection" function on the "DCS (Departure Control System)/check-in" side of Amadeus Altea.
As a result, you will still have 2 seperate PNRs on the reservations side, while one combined "customer record" on the DCS side.