Merging two Photos libraries?
Solution 1:
I use PowerPhotos from FatCatSoftware to merge my libraries. I've merged more than 12 libraries and 500 GB of photos.
Solution 2:
If you use iCloud Photo Library and first upload one library and then make the second library the system library, the first library will get downloaded into the second library (if that makes sense).
Solution 3:
I would export all the photos from the smaller library, into a folder. Then go into the larger library and import the photos. This keeps metadata and location info in place.