Combining 2 Photo Booth libraries

I am not sure if there is a way to merge the two libraries.

You can, however, recover the actual image files, since they are stored inside the library bundles.

To access the contents, control-click on the Photo Booth Library bundle, choose Show Package Contents

then you can COPY (do not move) the files to another location. The bundle include the original file as well as the modified files:

FWIW, using Photo Booth to manage a library of images is not so great. It does not allow for any of the features that are present in Photos - to have albums, smart albums, face recognition, etc.

I would suggest you import these images to the Photo application, and only use Photo Booth to er... take photos.

I have tried but it does not seem like there is any way to combine libraries for Photo Booth.

My suggestion is to use Photo Booth only to take photos, and organize them in Photos.

You could create an Album in Photos to group all your Photo Booth photos. That is, IMO, even better than keeping them in Photo Booth. There are zero organizational features in Photo Booth, and so much you can do in Photos. The only distinguishing feature that I see in Photo Booth are those "filters" - but do you really use them for anything other than an occasional practical joke?

If you do Show Package Contents on each of these libraries, check what is inside each package in the "Pictures" folder, not the "Original" folder.

I am not able to trick Photo Booth into "seeing" photos by putting them inside the bundle using the Finder.