iTunes. Menu. Edit|Preferences. Select Sharing Tab. Tick the box "Share my library on my local network"
Select either Share entire library, or, Share selected playlists (tick boxes as required).
Menu. Advanced. select Turn on Home Sharing. Enter your Account details (Username and password).
Do the same for your second machine and ensure under Menu|Store you "Authorise this Computer".
Log in to both machines on your "Home Network".
Open iTunes. The Libraries for each machine will now appear in the Shared area of iTunes (lefthand pane in windows).
Youcan now import/export between the two librariesas you wish, stream to each machine and stream from one machine to say a TV using Airplay by accessing the library of the second machine. See here for more details
http://support.apple.com/kb/HT3819
To Back Up.
Create a playlist called Backup (add date to name of file if you wish.)
Add whatever you want by going to your library and dragging the items required into the Backup listname holder. When you have finished. Right Click. You have the option to either Export or "BURN TO DISC"
You can also create a Duplicate which will enable you to keep track of what is in your original or last Playlist Backup.
Hope this now solves everyone's problems.