Firstly you're looking at high season, so whatever seats that were available on the few remaining flights that it is possible to redeem on were probably snapped up about 2 months ago.

Secondly, SQ seem to only release 1 F seat at a time so try looking for 1 seat, booking it and then booking you/your companion in whatever class of service is available on the same flight.

If you get really lucky, SQ may even release another F seat on the flight whilst you are on the phone to the ICC (it has been known). If not, you'll have to religiously check ANA for a seat to open up and pay the change fee to upgrade.

If you do a search there are loads of threads with the same advice