Terence, good additional information, thanks.
Here is my situation. Using iPhoto Library Manager, I have 4 separate libraries, total of 190 GBytes.
Also, using Aperture I have another library, 70GBytes. I'd like to have just one library for all of these photos.
My Aperture version is 3.4.3, and we're running iPhoto 9.4.2
The reason for this setup is that my wife historicaly used iPhotos for her family snaps, etc but doesn't need Aperture to do simple croping, etc. However, more and more she wants access to my Aperture edited photos and it would be easier for me to just have all photos in one location so that she can look for what she wants, in any library through iPhoto, and I can still use Aperture for my photography work.
In Finder, each library shows as an individual package (I believe that is the correct term). The four iPhoto libraries are on an external HDD, and the Aperture photos are on the internal HDD.
When I mentioned 'virtual' I meant if the applications (iPhoto & Aperture) would create the unified library by linking between these separate library elements (packages?), and combine them through data table references.
Reading your input, I'll use iPhoto Library Manager to combine the iPhoto libraries into one single package. Then, I'm assuming I need to use Aperture to make this unified library.
If I'm way off base, let me know, otherwise thanks again for pointing me in the right direction.