I've never done a fanmix before, but I'd love to give it a shot. My thoughts on how it might work as a multiple-people-bang are mostly:

An option: we set up a googledoc with character and/or pairing headings, and we all throw song titles and links to youtube/uploads under said categories, randomly as we stumble across them. This has the advantage of people cross-polinating, so to speak. Serious fanmixers might howl in horror, because it's harder to plan for a coherent sound that way, I'm guessing.

Another option: We either volunteer to do a particular set of characters and/or pairings, or pull them out of a hat and get assigned them. We're then in charge of finding that set, and will take someone else's character/pairing set to drabble up a storm with.

Actually, if we ran with option 2 in a googledoc, we could then offer each other songs on the side while still focussing on our main set.

This would also probably work for those of us who want to write drabbles first, and then offer them to people for song-finding?

I offer these options for them to be picked apart so we can put together somethings awesome :D Have at it!