You can only merge Photos Libraries by syncing them with iCloud Photo Library.
It would have been easier to merge the Libraries while still using iPhoto. The full version of iPhoto Library Manager can merge libraries, or if you have Aperture, you can merge iPhoto libraries with Aperture, but in Photos it is not supported.
To merge in iCloud Photo Library: If both libraries are small enough to be stored in iCloud given your iCloud data plan, you can enable one of your libraries as iCloud Photo Library, let all photos upload, then switch to your second Photos library and enable this library as your System Photo Library and enable iCloud Photo Library.
Now the photos from the second library will upload to iCloud and you will have a merge library in iCloud. Your second library will now have all your albums and edits, and the keywords, captions, titles. However, the faces from your first library will not sync back from iCloud.
See: Use Photos and iCloud Photo Library on multiple Mac computers - Apple Support