In addition to browsing your photo libraries in the main PowerPhotos window, you can also open individual libraries in separate windows. This lets you view different libraries side by side, and set up drag and drop photo/album copies between windows.
To open a library in a separate window, select the library and select File > Open Library in Separate window, or control-click on the library and select Open Library in Separate window from the menu that comes up. You can also option-double click a library to open it in its own window.
Library windows can also be combined into a single window with tabs for each library. Start by opening multiple libraries in their own window, then select the Window > Merge All Windows menu item. This will combine the separate library windows into a single window with tabs for each library.
Because the main window is still crucial for many functions of PowerPhotos, PowerPhotos will automatically quit when the main window is closed. If you like having libraries open in separate windows and want to keep them open between launches, open System Settings, go to the Desktop & Dock section, and uncheck the box labelled “Close windows when quitting an app”.