Merge 2 separate photo libraries into 1

It is not possible to import a photos library into another library to merge them. Photos can only import photos or videos.

You have three options:

1. Export all photos from the smaller library (as the unmodified original) and import the exported photos into your second library. But you will lose the adjustments, named faces, and any albums you created. See: Combine libraries in Photos - Apple Support
2. Merge the libraries with the paid, full version of PowerPhotos. This will give a very good, nearly lossless merge on macOS 10.15.1 Catalina or newer. Which version of macOS do you have installed?
3. Or, if you are using iCloud Photos, merge the libraries in iCloud by making both libraries your iCloud Photo Library, one after the other. This will give the best, lossless merge.

See: Notes on Merging Photos Libraries, 2019 Version

There are two ways to do what you want: 1 - using the iCloud Library; 2 - using the paid version of PowerPhotos.

The iCloud Library method would be the most complete but would require purchasing enough iCloud Drive space to house the two libraries and would a lot more time.

The PowerPhotos method is described in this PowerPhotos help page: Merging libraries.