One Library, double access
iTunes will not let you access the same library with multiple computers at the same time. Just can't happen. But there is no need to have to do that with the updates to home sharing. While I do not use a NAS server I do have all of my music on one hard drive and all of the movies on another. Both drives are shared on the network. These are the actual media files, not what iTunes refers to as the library. The library is just a data base that tells iTunes where to find the file you want to play. It also stores the meta data, (song name, artist, etc) in the xml file and the artwork in another. The itunes libraries sit in the home directories of each computer. Whenever I add anything to the library of one computer it adds it to the other, regardless of if it is music, apps or videos. It doesn't make a copy, just adds the location of the original file to the library.
So here is what I did to essentially sync the libraries between my old reliable G4 and my mini.
In iTunes preferences set one computer to "Copy files to iTunes media folder." This will be the master computer you always use when adding music files, etc. using the optical drive, another hard drive, thumb drive. The other computers you uncheck the copy box.
Make a copy of the itunes library, library xml and artwork files found under user name / music/ itunes of the master computer, place in a folder named itunes library copy, and then place the new folder in the second computer's itunes folder. Start up itunes on the second computer holding down the option key and select the itunes library in the new folder. Now you have two separate but identical itunes libraries running. With home sharing running on every networked computer, up to five, every time you add a music or video file to the master computer the others will add it to their library. You only have to do the library copying once.
The real beauty of this method is it still keeps separate the files for IOS devices. For instance my wife uses the G4 to sync her iPhone and Nano. I use the mini for my iPad and iTouch. They are sharing the same apps but the app data such as scores, email, preferences, etc are still kept separate.
One last thing, if you accidently add a music or video file to a computer other than the master, you can still use home sharing to make a copy to the master media folder. On the master computer, click on the shared iTunes library in the left column then click on Music or Movies or.... In the bottom right corner will be a button that when clicked will show the files missing from the master computer. Click import and it makes an actual copy to the media folder.
There may be other ways to do this with a NAS, but it works great for me. Each computer can be playing something different. Can have different movies or songs playing on each or on the ATV2, although when the ATV2 is using one of he computers to stream, that computer is locked out and dedicated to the ATV2
