The best way woud indeed be Aperture: Here is a knowlege base article by Apple explaining the procedure:
Aperture 3.3: How to use Aperture to merge iPhoto libraries
Aperture 3.3: Using a unified photo library with iPhoto and Aperture
But since Aperture is not exactly cheap, you might prefer not to invest into Aperture, unless you are considering to switch to a more advanced image management software anyway. Then that would be a very good investment. Aperture has more advanced image editing options than iPhoto and better organisational tools. It works well with very large photo libraries. You'll find some tutorials, the manual, and other help on the Aperture Support Page: Aperture Support
Another option would be the paid version of iPhoto Library Manager, but that would not be able to maintain all metadata.
Regards
Léonie