Sync iTunes library with Dropbox without letting iTunes organize the folders and files
You can put your entire iTunes library (not just the music files) into the Dropbox and create iTunes aliases in your ~/Music folder on both computers, as long as the paths are identical. This means you need to use the same user name on both machines. Ie., /Users/Username/Music/iTunes/iTunes Music. Then, when you add music to iTunes on one machine, when the library syncs over Dropbox, the music will be present on the other machine.