How do I merge 2 Photos libraries?

See the article https://support.apple.com/en-in/HT209528

There are two ways:

1 - with iCloud Photo Library is the most complete method.

2 - with the paid version of PowerPhotos as described in this PowerPhoto tutorial: Merging libraries. If you go this route be sure to migrate the smaller library into the larger and have backups of both before you start. Read the tutorial carefully because there are some caveats to this method.