iPhoto on your Mac does not have a tool to combine two iPhoto Libraries. If you have access to Aperture (Apple's professional photo application) you can use this to merge iPhoto libraries.
Without Aperture: Either you export your photos from one library (File > Export) and import them into the other (File > Import to Library), or use iPhoto Library Manager to merge the libraries. But you will need the full, paid version of iPhoto Library Manager for this: iPhoto Library Manager for Mac - CNET Download.com
A disadvantage of exporting and reimporting would be, that you cannot export the originals and edited versions paired - you would have to reimport both separately and have many seemingly duplicate images.