Here are the best options IMO. AC has some great Z fares to and from Asia, which will depend on finding Z inventory on AC and D inventory on SQ.

Both travelers would need to get to either NRT or HKG. From NRT or HKG you would take the SQ 380 flight to SIN. (There are 380 flights and non-380 flights - so choose carefully - and of course the plane could be changed!)

Do SIN-KUL-SIN on a separate ticket. All fares are dirt cheap. Take your pick in business or economy on these one hour flights from among SQ, MH, or JL.

Come back in reverse, through NRT or HKG.

This can all be done for under $5500 all-in, if, of course, there is Z/D availability on the AC/SQ flights they want. I had no trouble finding it on random dates I looked at.

You won't be able to book this at AC.com, and it may or may not be possible to book them online at all. It would depend on the ability of the online booking engine to let you pick exactly which flights you want. So they might want to use a travel agent.