Photos libraries

Hi

This is the officially documented way. If you choose this method you would probably want to merge the photos from the smaller library into your larger one.

This article describes how.

https://support.apple.com/en-us/HT209528

The problem with that is you can either copy edited versions or original versions - but if you copy both, then they will appear as duplicates in the library.

Alternatively, you can get the paid app (power photos) which will copy photos from one library to another, complete with metadata, edits, and albums.

Finally - you can use iCloud. You'd have to pay for a month of enough space for the combined merged library, but this will be cheaper than power photos, unless you want to have the facilities that power photos offers into the future.

To use iCloud you sync the first library with iCloud, then set the second to be the system library, and that will sync too, merging it's contents with the first in iCloud. Once you've done that you'll have both sets of photos in the second library, and you can stop using iCloud if you wish.

I second Tony's recommendation of PowerPhotos for merging libraries. But which system version do you have installed? Merging with PowerPhotos will give a very good result in macOS 10.15.2 or later, but not on earlier system versions, because the older versions of Photos did not make it possible for Power Photos.

PowerPhotos cannot merge optimised iCloud Photos Libraries. The merge will be incomplete. If you are already using iCloud Photos on your Mac and are using the "optimise" feature, merging the libraries in iCloud will be the best option. Also, PowerPhotos is continually hanging on my Mac with macOS 11.2 Big Sur.

You may want to have a look at this User Tip for merging in iCloud: Notes on Merging Photos Libraries, 2019 Version

Sorry - here it is

https://support.apple.com/en-us/HT209528

Somehow I've managed to put an error in the link without editing the text.

Leonie's user tip is better though.