The best way to organize a mix in iTunes is to create a playlist with all of the songs, make sure the album title is the same for each artist, then select all of the songs and open the Info menu (CMD-I) and mark them as a Compilation. Marking them as a compilation will organize them on the file system together under the "iTunes/.../Music/Compilations/" directory, which hopefully will make migrating off of iTunes in the future an easier thing to do.