I have two libraries in Photos on my Mac. When I browse to upload images to a photo-sharing website, or email, I can't locate the library that the newer pictures are in. It only shows one of the libraries. How do I get it to browse in both libraries? I don't see a way to choose more than one library.
Photos can have any number of Libraries, but by default, it opens the last open library when you click on the Photos icon. To open other Libraries, hold down the Option key when clicking on the Photos icon. This will pop-up a dialog, showing you the available Photos libraries.
Photos sync's new photos from iCloud (say from your iPhone or iPad) to only one of the libraries, and that library must be declared as the System library. Perhaps you are not seeing new photos because the library that is opening is not the System Library.
You will see which Library is the System Library when you hold down the Option key and open Photos: that library will have 'System Photo Library' next to it. This one will have your new photos in it.
If you want a different library to be the System Library, simply open that other library (as mentioned above), then in Photos, choose Preferences>General> Use as System Library. This will cause your iCloud photos to sync with this Library.
There is no a way to browse all images across all libraries, each are separate. Photos does not have a way to open multiple libraries at the same time. If you only want one library, you can simply import photos from the other library into your System Library, by using the Import function, and navigating to the other Photo library, typically found in your Pictures folder.
The photos picker that is part of the open file dialog only looks at the system library which is typically the current one.
You can open photos and switch libraries, set up a script to automate at process or consolidate your photos into one library - each has drawbacks, but hacking the photo picker is worse than the alternatives I've presented.