You can share your music library between the Mac OS 9 and Mac OS X versions of iTunes 2 by making an alias of the iTunes folder, inside your Documents folder. The iTunes folder contains the iTunes Music Library file and the iTunes Music folder with your encoded songs.
Important: These steps do not work with iTunes 3. You cannot share music libraries between Mac OS 9 and Mac OS X if you use iTunes 3.
To use the Mac OS 9 music library with Mac OS X:
1. Drag the iTunes folder that may already exist in the Documents folder of your Mac OS X home to the Trash.
2. Make an alias of the iTunes folder that is inside your Documents folder on your Mac OS 9 startup disk.
3. Drag the alias to the Documents folder that is inside your Mac OS X home.
4. Rename the alias from "iTunes alias" to "iTunes".
Now, when you use iTunes for Mac OS X, it will follow the alias and use your music library from Mac OS 9.
To use the Mac OS X music library with Mac OS 9:
1. Drag the iTunes folder that may already exist in your Documents folder on the Mac OS 9 startup disk to the Trash.
2. Make an alias of the iTunes folder that is inside the Documents folder in your Mac OS X home.
3. Drag the alias to the Documents folder that is on your Mac OS 9 startup disk.
4. Rename the alias from "iTunes alias" to "iTunes".
Now, when you use iTunes for Mac OS 9, it will follow the alias and use your music library from Mac OS X.