I have had combinations of paid and reward tickets when flying with my family. I found it useful to call SQ reservations and ask them whether we could be seated together (Y class - no seat selection for reward booking!).

Before calling them I made sure that for the paid tickets, which I had booked on line, I had picked the seats I preferred, with some empty seats next to them. Eventually, my family members were assigned the seats next to those I had picked. But no guarantee, you will only find out upon checking in...

I agree that this may be less useful if there are just two pax, but then again it increases the chance to get your preferred seats.