Leonie,
What is different about Aperture that doesn't allow libraries to be shared?
iPhoto Libraries have a feature that allow you to share your library to other iPhoto installations on the same network
iPhoto & Aperture both can use the same library/database
Aperture can share its library to Apple TV, iPhone, & iPad users via Home Sharing on the same network
I recently switched from iPhoto to Aperture, and I really miss this feature. I have a main repository of photos residing on my Mac Mini. When I was using iPhoto, my wife could access these photos via a Shared Library within iPhoto. But now that I am using Aperture, she cannot access the same files unless I close Aperture and reopen the same library in iPhoto.