Just done this recently albeit to SYD. Booked outbound online using the "use 241" option on the booking. At that point I secured 2 F seats to SYD outbound only.

14 days later when return availability opened up I rang You First and said "I have a 1st class booking, please can I add the return sector and use my 241". No problem they said and I got my CW tickets to SIN and Y tickets from SIN to LHR. Alas there was no other availability and I had to pay a £25 fee per ticket for the change, but given what I secured I thought this was fine.

At no point did I believe that my outbound seats were in jeopardy while booking the return (although they might have been), so hopefully you can do the same.