Unfortunately for you, two hearts is when Pokemon receive 1.2x more EXP, and there isn't a way to evolve into Sylveon differently.
Source
However, if you get an Eevee from a different OT and evolve it, then trade it away and back again, the affection will reset because Pokemon only remember affection/happiness from their OT. If you have more than one DS and copy of SM/USUM, then this should be no problem at all. This I know from experience.