I don't take a lot of photos, and don't use Aperture, but when I stumbled on this Apple tech note, I had a feeling that it might be useful to those who take a lot of pictures.
Apple explains how to use a "unified photo library with iPhoto and Aperture." Setting up the unified library isn't rocket science, but the document lists a number of limitations that are good to know about. If you use both programs, read this document to see how to share a library, and to see what you can and cannot do with each program.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20120626072023264