Photo

The best way to merge Photos Libraries is to upload them to iCloud Photo Library, one after the other. The merged library will download to your Mac.

Merging in iCloud requires a paid subscription - consider to upgrade your storage plan just for a month. But it has many advantages: iCloud Photo Library will resolve conflicts between duplicate originals and store the original only once, when it is merging the libraries. All edits you applied in Photos will be retained in a lossless way. When the merged library downloads to your Mac the edited versions and the originals will still be paired nd you will be able to undo adjustments individually or revert to the original. If your libraries are large, the upload may take several days, however.