Yes, it's very easy, as a matter of fact: Aperture allows two different ways to manage your photos -- which can be mixed at your own discretion, too!
(1) Managed Library: all photos get imported into the Aperture Library bundle. It creates, manages and deletes them if they are changed in the Library, etc.
(2) Referenced Photos: Aperture leaves the photos at the current location (they can be on your local drive, on an external drive or on a network drive).
As I said, you can switch on a photo-per-photo basis whether you want to have them managed (in your Library bundle) or referenced. To save yourself a few clicks, you should change the preset in the prefs, though.
Referenced photos are a little more work-intensive: if you move them in the Finder, you need to reconnect the pictures in the Library, too. If you are very much fond of managed libraries, you should get a bigger internal harddrive (which is what I did).
Edit: There is even one more solution if you prefer managed libraries: you can create libraries on both drives and then open the respective library by double-clicking it. Keep in mind that then you don't have access to the thumbnails on the external library if your external harddrive isn't connected.