Can not so fast counter Singularity attacking two different units?

Yes, Not So Fast can counter Singularity.

According to the card text for Not So Fast, it can counter an enemy spell or ability that chooses a friendly unit or gear. As established by the official rulings for Singularity (FAQ #8662, #3483, #7180, #8602, #5969, #2714), Singularity targets units. Because Singularity requires you to select units to deal damage to, it meets the "choosing" criteria required by Not So Fast.

Note: I could not fully verify this ruling against the retrieved rules.