Interinmente is right, just add a new entry into the Slingbox Directory.
The second (or third, fourth, fifth...) Slingbox should be completely set up and working first. After that, all you need is the SlingID of the new Slingbox and it's as simple as adding an entry.
Here's how you can find your Sling ID:
If you add a new Slingbox and can't watch it over the internet, you almost certainly need to fix the port forwarding on the router at the Slingbox location.