Saver comes from O and Advantage comes from A for First. However, there is never a guarantee. One takes the Saver and then the Advantage should not be affected, but this is revenue management and not algebra. So when there were 2 and you took one it could be 1 remains, it could go to Zero, with the Advantage disappearing, heck if you are super lucky a second saver gets released (highly unlikely, just for illustration ).

Something like O1 A1 F4 doesn't mean 6 seats are on offer (a 77W has 4 F seats btw), it means SQ is willing to sell one seat at O (saver), one seat at A (advantage or discount First) and 4 seats at F (Full fare First).

Now this being JFK-FRA-JFK, I think it will work grabbing Saver and then the Advantage as F is more busy between SIN-FRA-SIN usually, good luck.