Same iTunes Library between networked Mac and PC
Basically using iTunes on a shared storage is really bad. Even if you get it running as you want now, you need to reconnect to the network storage before launching iTunes EACH time (and never open iTunes if not connected to the network) otherwise iTunes standard behaviour is to recreate a new iTunes library locally and then you're stuck there again.
However:
Your current problem is most probably that you are using the same iTunes library file/database on both machines (residing on the network volume). There are two parts to your iTunes Library: The iTunes Library Database (.itl and .xml files) and the Media itself. You want the library database to stay in it's default location on your local drive and You Only want the Media folder to be on the NAS.
To achieve this you start up iTunes with a empty library (in the local Music folder) on EACH machine then import the music from the network folder. But before importing make sure you change the setting &quot;Copy files to iTunes media folder when adding to library&quot; to OFF! Otherwise your data is copied back to the local machine...
Then never start iTunes with your network storage disconnected and you will be fine.