iphoto library

Are you really asking about iPhoto and not Photos for Mac?

• In iPhoto you can switch between libraries by by quitting iPhoto and then double-clicking the iPhoto Library you want to open.
• The same will work in Photos for Mac. Quit the Photos.app, then double click the Library to open it in Photos. Or hold down the options key ⌥ while launching Photos and select the library from the Library Chooser dialog: Create additional photo libraries in Photos on Mac - Apple Support

To create additional libraries in either of the two applications, hold down the options key ⌥ while launching the application and select to create a new library.

Photos Libraries need to be on a volume with the correct file system format and with a wired connection, not a on a network share or in a synced folder like DropBox. Move your Photos library to save space on your Mac - Apple Support

What is the issue you are trying to manage?

If it is any more than just choosing which library to use, check out the (paid) iPhotoLibraryManager from fat cat software.

Launch Photos with the option key held down and point it at those libraries.