I have the Led Zeppelin album Physical Graffiti, which is a double album. The way I tell iTunes that it's one album is like this: All the tracks' album name is "Physical Graffiti" (no disc 1 or 2 in the album title). Disc 1 is 6 tracks, and Disc 2 is 9 tracks, so in all the tracks I make them 1 of 15, 2 of 15, etc., but 1 of 15 through 6 of 15 are all marked as disc 1 of 2, and tracks 7 of 15 through 15 of 15 are marked as disc 2 of 2.