There are several ways to merge two or more Photos libraries, each with advantages and disadvantages.
If you have plenty of Internet speed, high or no data cap, and enough iCloud space, you can use iCloud Photos by successively connecting each library to iCloud Photos, waiting for it to upload all of its photos and then download any that were previously in the cloud, then do likewise with the other library(libraries). Whichever library you do last will contain the merged photos. This method has the advantage that it maintains album organization and the ability to revert previous edits. The disadvantage is that it takes a long time and uses a lot of network capacity.
You can do it more quickly locally by opening the old library, exporting photos to a temporary Finder folder(s), switching to the new library, and importing from the temporary folder(s). You have to choose whether to export the original photos (losing any edits) or export the edited photos (losing the ability to back out the edits). Maintaining folder and album structures can be tricky, but is possible for simple structures. The advantage is speed. The disadvantage is the need for a lot of temporary working disk space and having to choose what to do with your edits.
There are also third-party utilities that can bypass the need for both iCloud space and local temporary disk space by copying photos directly from one library to the other, but they require making the same choice between losing edits or losing the ability to back out old edits. I'm not sure how good they are about preserving folder and album organization.
