With PowerPhotos you have these options in merging one library into another:
Also copied will be the metadata including keywords, descriptions, titles, dates, and favorite status.
When I've done it I've picked the smaller library with fewer projects to copy into the larger of the two libraries. Also make sure you have current backups of the libraries just in case.
The iCloud Library method is a more complete merge but will take longer and you'll have to purchase enough iCloud Drive space to hold the merged libraries.
PowerPhotos has a demo version which will do everything except merge libraries and search for duplicates.