If you have iPhoto libraries on two separate computers and would like to merge them together into one library, the first thing you need to do is choose which machine you'd like to use to perform the merge (we'll call this the "merging machine"), and make both libraries accessible from that machine. This can be done a few different ways, including:

• Using file sharing to connect one machine to the other, so your merging machine can access the other machine's library over the network. Note that iPhoto requires read/write access to open up a library, so connecting as a guest user or another user that can only read, and not write to, the library will probably not work.
• Rather than using the library directly over the network, you can instead first copy it to the merging machine's hard drive, and then perform the merge using the copy of the library instead. This is the recommended approach, assuming you have enough free space
• Copy the library from the other machine onto an external hard drive, then connect that hard drive to the merging machine.
• Put the other machine into target disk mode and connect it directly to the merging machine

Once you have the library (or a copy of the library) accessible to the merging machine, next add the library to iPhoto Library Manager's library list. Once both libraries are visible in iPhoto Library Manager, you can the merge the libraries like any other libraries on your machine. If you would then like to use the merged library on both machines, simply copy the merged library back to the other machine and add it to iPhoto Library Manager so you can use it there as well.

See Also:

Dealing with different versions of iPhoto

Copying an iPhoto library between computers

Storing a library on an external drive