In Aperture, you can change the active library you are using for importing and editing pictures. Changing this library will change which photos are displayed in the Photos pane of iMovie, iDVD, and the Desktop Pictures and Screen Saver system preference panes.
To access a different Aperture Library:
If you don't already have an additional Aperture Library and want to create one, navigate to the folder where you want to create the library at step 3, and then click Select.
By default, Aperture creates a new Library file named Aperture Library. It’s recommended that you rename Library files to avoid any mix-ups or the possibility of overwriting one Library file with another accidentally. After renaming a Library file, make sure to open Aperture and select the renamed Library file.
When you have multiple Library files and you want to switch between them, you use the Preferences window in Aperture to select the Library you want. You then quit and reopen the application to complete the switch.
Additional information is available in the "Working with Library Files" section of the Aperture User's Guide.