I’ve been asked a few times over the years if it was possible to merge multiple iPhoto library files into one large library. The answer is yes, but it’ll cost a few bucks.
iPhoto Library Manager is a handy tool for anyone who has large iPhoto collections
iPhoto Library Manager ($19.95) lets you easily create and keep track of multiple iPhoto libraries, but that’s only the beginning. It also has the ability to copy your photos from one library to another, while keeping track of photo metadata that is normally lost when exporting from one library and importing into another.
You could split or merge library files manually, but that could result in a lot of time spent dragging out and re-importing a lot of photos – and it doesn’t guarantee all your meta-data will make the trip. If you take a lot of photos, and you use iPhoto to manage them, iPhoto Library Manager will make organizing and backing up those precious photos much easier.