Tara is far too high, although I've never really tried to sing either part since I'm not particularly keen on Standing OR Under Your Spell, never mind the medley, and Buffy is the obvious one for Walk Through the Fire since she's got the most lines. I had to go with Captain Hammer here, though.

The only problem with harmonies, is choosing which to sing along to. I really hope Fillion doesn't steal every scene, because NPH is equally brilliant, and I'd love for him to get the best lines sometimes.