Yes, you can backup multiple PCs using the WD Smartware. You need to make sure that all of PCs and MyBookLive are all on the same home network. Make sure that each PC can see and detect it. One sure way is to run the Installer CD on each PC and map the network drive to a drive letter.
From you 2nd PC, try this.
Launch WD Smartware
From the Home Tab on the top row click on the MyBookLive unit (you can you have selected it by the blue halo around it)
Once you have selected it, then click on Backup Tab.
PS: Make sure that each PC you have has a different computer name assigned to it. If all your PCs all have the same name, then that could cause it to see the first computer that was setup only.