Can photo libraries be merged?

Download and use PowerPhotos.

(142302)

Two ways -

1 -use iCloud Photo Library and set one library as the system library and after the upload is complete set the next library as the system library - this will merge the libraries - once the merge is compete and you have verified everything you can delete the first Photos library and you can archive and delete the two iPhoto libraries

or

2 - use PowerPhotosto merge one version of the photos

LN

Question is, is there a way to merge them together so that they are all one library again?

To elaborate on Larry's advice:

It will depend on whether your Photos have edit and metadata applied that you need to save when you merge the libraries which option to choose.

You will want a lossless merging, if your photos have been edited or you applied titles, captions, etc. Lossless means, all metadata will be applied, all albums and folders preserved, and both the original and the edited versions will migrate paired as original and version, so you can undo the edits without losing quality. Aperture could merge iPhoto Libraries this way, but neither iPhoto nor Photos can.

There is no lossless merging of Photos Libraries other than iCloud Photo Library, and even the merging in iCloud will not include the faces albums and the print products.

1. Uploading both libraries one after the other to iCloud will merge the libraries in iCloud. It will be like using iCloud Photo Library from multiple Macs, see: Use Photos and iCloud Photo Library on multiple Mac computers - Apple Support
   Your albums and folders will migrate, keywords, titles, and other metadata. All edited images will be paired with their originals, so you can undo the edits and revert to the original. The searchable faces names will upload, but not the faces thumbnails and albums. Photos will scan for duplicates while merging. It is the best way to migrate libraries you invested much work into, but uploading large libraries to iCloud requires a paid subscription for more storage than the free 5GB - for at least a month, and it is slow. My library with 40000 photos took a full week to upload.
2. Merging with PowerPhotos is faster (https://www.fatcatsoftware.com/powerphotos/). It will migrate the metadata and the albums, but you have to decide, if you want to use the originals or the edited versions. So you will either lose the editing work or the high quality originals or create redundancy by merging twice in two passes, once to transfer the originals, and then the edited versions. You will have to add a pass to remove duplicates afterward. See chapter six in the manual: https://www.fatcatsoftware.com/powerphotos/Help/merging%20libraries.html

A third option would by to keep both libraries separate and use PowerPhotos to browse the libraries and to transfer selected albums as you go.

Thank you for this detailed advice!

About your comment: There is no lossless merging of Photos Libraries other than iCloud Photo Library, and even the merging in iCloud will not include the faces albums and the print products.

What do you mean by Faces albums? I noticed that Face names did not appear on photos when I exported them and imported them, nor when I used Power Photos. Is that what you mean by Faces Albums? In other words, keywords are preserved, but Face metadata is gone? Or does the iCloud merging technique preserve the face names on photos, but just not the "faces albums" (not sure what you mean by that). Thanks again.

What do you mean by Faces albums? I noticed that Face names did not appear on photos when I exported them and imported them, nor when I used Power Photos. Is that what you mean by Faces Albums?

When you name faces in Photos, the Faces album will show you the faces tiles - a grid of thumbnails with the key face for each person you named, like this:

If you click the thumbnail, you will see an album with all named faces of this person.

When you sync your library to iCloud each photo will keep the name associated with the photo, so you can search for the persons name on the other mac, but the thumbnails with the position of the face on the photo will not be synced. In the example above you could search for "Becky" on all all macs syncing with iCloud and find all photos of Becky, but the other Macs will not show the faces tiles. The position of the circles around the faces does not sync to iCloud and does not sync back from iCloud if you merge libraries this way. (Tag faces in Photos to find people on all your devices - Apple Support)

PowerPhotos cannot preserve the faces data either. If you export a photo with named faces, the face name will be added to the photo as a keyword. If reimport the the photo, you can search for the keyword. You need to enable these options in the Export panel, if you export a photo:

You are brilliant, and I have noticed what you are describing but couldn't verify. Thank you so much for all your help!

You're welcome 🙂