Of course, Filly could also be Faith/Willy. There's a pairing I don't think I've ever seen.

I don't think I've ever seen one for Willow/Faith, so it's up for grabs. Then again I'm not a huge fan of smushnames in the first place (though I'm still rather pleased that I managed to tag one story "Spanythingthatmoves"). I guess Willaith is as good as it gets unless you want to get creative and go with, say, Willehane. (Also, see Deird's post on pairing names, which gave the world Wooth, Weslared and Bunnewiffaithedy.)