Over the years I've tried and documented several hacks. None are adequate. I tried a merge with iPhoto Library Manager a while back and got some bad results, but I'm giving it another go with the latet release. IPLM has to install an evil InputManager/iPhoto Plug-In to make this work, so it's a non-trivial task. (InputManagers are the TSR*s of OS X.)
I'm only in test mode. I've copied Libraries of a few thousand images to an external drive and I'm merging them and testing. It will be a while before I merge for real -- I might buy Aperture first. So far, when merging iPhoto 6.02 Libraries, it's working better than before. A few notes:
1. It doesn't import Books, Calendars, Slideshows, etc. Those are lost. Same problem with Aperture. You can print Books and Calendars to PDF as an archival approach.More testing to come, I'll update this post.
2. It changes smart albums to regular albums on import. Not a bad idea.
3. It does handle keywords, it will merge those that match. Keywords are royally messed up using older techniques.
4. You can either merge Albums of the same name or create new ones. I create new ones.
5. If there's an Original and Edited non-RAW image it imports both. (Not sure about RAW though, that's handled differently.)
6. Folders aren't copied, you just get the albums.
7. Rolls are preserved and roll names and comments are imported on merge.
8. Albums are preserved as are titles and comments on images.
9. My iMac is really working to get this done, fans going full blast. Good workout! I let it run overnight.
10. I've set the option to quit and restart iPhoto every 20 rolls. I don't trust that thing.
* Wow, I'm old. Wikipedia doesn't even define what a TSR is. "Terminate and Stay Resident" assembler programs were MS/PC-DOS hacks that allowed things like the original Sidekick (not a piece of hardware) to work. They were notorious for causing crashes.