combining libraries in photo

There is no built-in way to merge or combine Photos libraries. The solution Siabh offers isn't combining libraries, but rather moving some of the contents of one library to another - that involves inevitable dataloss as you're forced to choose between your originals and edited versions, and you lose the edit history and so on.

You can merge libraries with PowerPhotos, among many other features.

https://www.fatcatsoftware.com/powerphotos/

I second TD's recommendation to use PowerPhotos to merge libraries. But it will only be a nearly lossless merge, if you are running macOS 10.15 Catalina. Before macOS 10.15.1 it has not been possible to move the edited versions of your photos paired with the originals, so you can revert the adjustments individually after merging the libraries. Also, Power Photos cannot move items between libraries if you are using iCloud Photos with "Optimise Mac Storage".

If you have an older system version or are using iCloud Photos with Optimise Storage, consider to merge the libraries in iCloud. Syncing both libraries, one after the other, with iCloud Photo to will merge the libraries in iCloud and the merged library will download from iCloud back to the currently used iCloud Photos Library.

To combine two libraries, say from library A to library B, the steps will be

• export pictures from library A
• import pictures into library B
• delete library A (if required)

Read how to do it here: Combine libraries in Photos - Apple Support

Hope this helps, otherwise write back giving us more details!

Good Luck!