This in-depth video walks through a few different options of how to merge Photo Libraries together. Lots of different options depending on your situation.

If you're using iCloud Photos, I highlight four different ways to merge photo libraries:

Option 1: Exporting and Importing

Option 2: Using Finder and Package Contents

Option 3: Using third party app PowerPhotos

Option 4: Using iCloud Photo

*Always make sure you have a backup of your Photos Library. If you're attempting a merge using the iCloud Photos method, also make sure that you have it set to download all the original files prior to merging and verify that you have everything on a hard drive!