Merging Photos libraries from two different Macs

The most complete way is to enable the iCloud Library on each Mac and let both of them sync to the iCloud Library and each other. This will include projects, faces, etc. It does require purchasing sufficient iCloud storage to house both libraries. If you don't have enough space on your portable device you can also enable "Optimize Mac Storage" on it. And it will take some time to complete the syncing depending on the sizes of your libraries.

If you use PowerPhotos to merge the two libraries both devices will have to be running the same system and Photos version. With the iCloud Library that's not a requirement.

Hi,

The best tool for merging your two libraries and retaining all of your organisation/album structure, before/after edits, etc in my experience is Power Photos from Fat Cat software. Do you have to pay for it unfortunately, but its well worth it (I have nothing to do with the company BTW) and the developer is very helpful. The Duplicate Photos feature is very good also.

Alternatively, if you didn't want to pay for the software, you could just export all of the photos from one library and re-import into the other but you are likely to lose your organisation and edits. Depending on how many photos you have you could export an album at a time, but you will need to decide whether you export the photo pre/post edit. This may not be issue if you have not spent a great deal of time editing your photos.

Once you have created your single, merged library, you mentioned that you only wish to import your any new photos once. The best way to do this is with iCloud Photos. You set one library as the master (via preferences) and any changes you make to either the master library or the secondary library are synced to the other. You mentioned that the iMac is quite old, which version of OS are running as they may impact whether iCloud Photos will work.

You are right, Russ. PowerPhotos is really good at merging libraries, as long as we are running macOS 10.15 Catalina or newer. With th older versions of MacOS X it will not be possible to move both the edited versions and the originals between libraries, and the photo edits will not be reversible, see: https://www.fatcatsoftware.com/powerphotos/Docs/merge_libraries.html

And PowerPhotos cannot merge optimised iCloud Photos Libraries. PowerPhotos needs all items to have been downloaded from iCloud to be able to merge libraries. To merge two libraries residing on two different Macs I would simply upload both libraries to iCloud Photos and they will be merged perfectly in iCloud as recommended by OT.

See: Notes on Merging Photos Libraries, 2019 Version

Uploading the two libraries to iCloud will merge the libraries. iCloud Photos will solve conflicts between duplicate photos and albums. That is why the upload into iCloud will be slow, because of the duplicate checks.

It is unlikely, that you will have duplicates in the iCloud Library after uploading and merging the libraries, as long the duplicates between the devices are exact copies.

I bow to Old Toad and Leonie's superior knowledge, they are right re the need for the same OS version if you were to use Power Photos. Hopefully iCloud Photos will take care of the duplicates. If you do end up with numerous duplicates for whatever reason, there are plenty of threads on here as to how to tackle the problem - most of them written by Old T or Leonie!

PowerPhotos will not be an option for you. You'll have to go the iCloud Library route.