Aperture and iPhoto now share a unified library format - iPhoto can open libraries created by Aperture and vica versa, see: Aperture 3.3: Using a unified photo library with iPhoto and Aperture
You can use Aperture to merge your iPhoto libraries into one new single library by importing your iPhoto libraries into an Aperture library.This way you can transfer all metadata tags and edits. Also you will not create duplicate images by importing edited versions and original images. See: Aperture 3.3: How to use Aperture to merge iPhoto libraries
This requires, that iPhoto and Aperture have been updated to the latest version. Open each Aperture library in Aperture 3.4.3 to upgrade, and each iPhoto library in iPhoto 9.4.2.
It is advisable to repair each library before merging (Repairing and Rebuilding Your Aperture Library: Aperture 3 User Manual, iPhoto 6 and later: Rebuilding the iPhoto library), also to unhide all hidden photos in the iPhoto libraries. These cannot be unhidden in Aperture.