Well, you need to book well ahead, not 15 days. Secondly, I don't ever need to fly to the US, only the EU. And getting 2x F seats between FRA and SIN has always been easier for me than 2x C seats on this route.
Good luck dealing with US routes though, sorry I can't help...
Don't you know that LH only releases F award to star alliance partners 15 days before the flight, SQ including? You can't book earlier if you wanted to. That's why I asked how you managed to do it. Also I have never seen two LH F on the SIN-FRA route available for award booking before.