If you have i Photo 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.
To merge the libraries: The library you started with in step 2 above includes content from all the merged libraries.
The other libraries are still present (and useable) on your computer.
Once you have confirmed that the unified library is working correctly, you may wish to remove the other libraries from your hard drive.
It is a good idea to back up all your libraries before doing so.
Once both libraries are visible in i Photo Library Manager, you can the merge the libraries like any other libraries on your machine.