One of the primary uses of Photos is to import and organize photos you have taken with your digital camera. There are a couple things you should know about how this works when you have multiple Photos libraries.
If you have Photos set up to launch automatically as soon as your plug your camera in, this will cause Photos to open whichever library you had last chosen to work with in Photos. This can sometimes cause the wrong library to be opened up, at which point you have to quit Photos, go open up PowerPhotos, switch to the correct library, and then open up Photos again. If this happens to you a lot, you might want to instead set PowerPhotos to open up when you attach your camera rather than Photos. This way, you can be sure that you have the right library selected before opening up Photos, and avoid importing photos into the wrong library.
To set this up, open up the Image Capture application located in your main Applications folder on your hard drive. Plug in your camera, and it should appear in the “Devices” section. Select the device, then use the pop-up menu at the bottom of the list and select “Other…”, at which point you can select PowerPhotos in your Applications folder, and click OK. Now, when you plug in your digital camera, PowerPhotos should automatically open instead of Photos. Note: on some systems, even after selecting PowerPhotos to open, the pop-up menu may still read “No Application”, but PowerPhotos will still launch when plugging in the camera.