This is quite simple and similar to creating a new library to begin with.
Now that you have multiple photo libraries, you’ll likely want to switch between them sometimes. Switching Between Libraries in Photos App for OS X Having a unique library for each work, personal, private, for example. There does not appear to be a limit to how many new photo libraries you can make in the Photos app for Mac, but juggling between tons of them could become unwieldy, so it’s usually best to make a new library for specific purposes. That means it will not contain any of the previously imported pictures unless they are added again. Remember, this new photo library is separate from whatever the default photo library was, whether it was set up as new or imported from iPhoto or Aperture.
How to Make a New Photo Library in Photos for Mac OS X This allows for completely different and unique collections of images. To be completely clear, making a new photo library means none of the existing libraries images will be included in the new picture library, unless specifically added. No, you don’t go to the File menu, where you can create new albums and folders for pictures, and is another reasonable management method, but instead you must use a key modifier while the Photos app is launching. Making a new Photos library is really quite simple in Photos for OS X, but it’s not entirely obvious.