PhotoStructure is designed for million-asset libraries, so you shouldn't need to switch between libraries in most cases.
If you want to have a couple separate libraries, though, you certainly can.
Click the tray icon, and pick "Preferences...". Scroll down to the Library path, and enter a new (or previously existing) PhotoStructure Library directory.
When you click "Save", PhotoStructure will close the currently-open library, and open a new library in that directory.
Keep in mind a couple things:
- Opening a new directory will not copy over prior assets. If it's a new directory, the library will be empty.
- If that directory had previously been a PhotoStructure library, all the assets in that library will now be visible.
- If both libraries are set to import from all drives automatically, you'll end up copying every one of your photos and videos twice.
Photo by Ross Findon