Is there any chance you can upgrade your Mac to a more recent system version? On El Capitan or options to merge two Photos Libraries are limited and lossy. Basically, you can do no better than to export the photos from one library and import them into the other library, as suggested by Apple: Import photos from another library in Photos on Mac – Apple Support (UK)
This will transfer the photos, but not the albums and folders and the detected faces, and the edited versions of your photos will no longer be paired with the originals as one photo. You can do better, if you have iCloud photos enabled. Then you can merge the two libraries by uploading them one after the other to iCloud. This will preserve the the adjusted photos as a pair, so you can revert adjustments to the original version, and also the albums and folders. See: Notes on Merging Photos Libraries, 2019 V… - Apple Community
PowerPhotos 1.7.7 or newer on macOS 10.15 Catalina or later can give you a nearly lossless merge. It will migrate the metadata (but not the faces) and the albums. Fully supported: photos and videos, keeping keywords, descriptions, titles, dates, favorites, locations, bursts, and Live Photos intact; albums, folders, and moments. On older system versions (up to Photos 4 on Mojave) you have to decide, if you want to use the originals or the edited versions. So you will either lose the editing work or the high quality originals or create redundancy by merging twice in two passes, once to transfer the originals, and then the edited versions. You will have to add a pass to remove duplicates afterward.
If you are cannot upgrade to a newer system version, I would keep the two libraries separate for the time being and simply switch between the libraries. Sooner or later you will need a new Mac anyway and that will come with a new system version, that supports better tools for merging your libraries.
