I have two libraries from the Photos app (version 1.3 on OS X 10.11.2).
Is it possible to merge them or to move pictures from one library into another?
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I use PowerPhotos from FatCatSoftware to merge my libraries. I've merged more than 12 libraries and 500 GB of photos.
The only Apple merge of libraries appeared in Aperture. You could use a third party option like iPhoto allowed, but the main method of merging is to export the photos in whatever organization you want (album oriented) on one Library to the filesystem and then import those into the remaining library.
This loses ratings, albums, faces, etc... but not geolocation and edits if you are exporting the edited photos and not the originals.
I have had limited success putting photos into iCloud Photo library and then deleting that local library after switching the other library to be the "system library" but I did have about 3% duplicate photos - so that was several dozen items to find and prune by hand. I wouldn't recommend this since it's not easy to fix iCloud library if it messes up badly. For what it's worth, my cloud library is now 80 GB and 20k items - so unless you are much smaller or larger, you could try that after backing things up.