Can photos be stored in multiple libraries?

Multiple Photo libraries have been possible in Photos since it was first released in 2015 and in the iPhotos app before that.

For information on multiple libraries, see: Create additional photo libraries in Photos on Mac - Apple Support

and

System Photo Library overview in Photos on Mac - Apple Support

Most of us use multiple libraries-- some archive pictures that won't be changing, and maybe one Library has our Favorites in it that we connect to iCloud to sync with our phones and iPads. Only one of the Libraries on the Mac can be synced with iCloud. And phones and iPads can only have one Library.

Many of us use the 3rd party app  PowerPhotos ($30) to switch between libraries and make other operations more manageable.

What has changed is that you can choose part of your Library to appear as a separate shared Library. It isn't really separate, but in some ways it acts like two Libraries, one of which can be shared with others.

Set up or join an iCloud Shared Photo Library in Photos on Mac - Apple Support