I use a MacBook Pro for daily use and have a Mac Mini as my media center. While iTunes has a lot of great functions (I love Genius), it does not have a simple way to have to standalone iTunes libraries and sync songs between the two. Sure you can share the music while you’re on the same LAN, or over the Internet (eg. http://www.simplifymedia.com/index.html), it does not have a simple function to synchronize a library between two computers. The following article describes how to use rsync to keep two libraries exactly the same.

http://guides.macrumors.com/Keeping_2_iTunes_Libraries_in_Sync

I found a second way to synchronize the songs that were missing on the iTunes library on my MacMini from the iTunes library on my MacBook. I used the following steps:

  1. Share the iTunes folder on the source (/Users/username/Music/iTunes)
  2. Mount the iTunes folder on the destination
  3. Open iTunes and select the ‘Add to Library’ option
  4. Select the mounted iTunes folder

iTunes now automatically selects the missing songs and add them locally to the library. This may take some time.