Photos in your System Photo Library are available in apps like iMovie, Pages, and Keynote. You can also sync them to iOS devices and view them on Apple TV. And if you want to use your own images as your desktop picture or screen saver, the images need to be in your System Photo Library before you can select them in System Preferences.

I’m finding that there are photos EVERYWHERE on these two machines—Photos library, iPhoto library, Photo Booth library, multiple file folders on dozens of different levels and in countless.

But two significant criticisms of iPhoto are frequently heard: poor performance with large photo Libraries, and a lack of easy-to-use support for multiple Libraries (the latter perhaps a necessity.
Hold down the Option key, and then double-click on the Photos app to see the libraries you have on this device Now you can move easily from one photo library to another and open the one you want by double-clicking it How to create a new Photo library on Mac? Creating a new library is easy.
You can't merge libraries using Photos; you can only view them separately. If you decide at some point you'd like to pick a different photo library as your default library for Photos, you can do so.

If you have only one photo library, then it's the System Photo Library. Otherwise, the first photo library that you create or open in Photos will become the System Photo Library. If you have more than one library, you might need to designate a System Photo Library, so other apps can access the photos and videos you want them to use.

Follow these steps to designate a System Photo Library:

Quit Photos.
Hold down the Option key and open Photos. One of the photo libraries is already designated as YourLibraryName (System Photo Library).
Choose the library you want to designate as the System Photo Library.
After Photos opens the library, choose Photos > Preferences from the menu bar.
Click the General tab.
Click the Use as System Photo Library button.

If you open a second or different library in the Photos app, and you haven't designated it as the System Photo Library, other applications will use photos from the original System Photo Library. Hold down the Option key when you open Photos to see which library is set as the System Photo Library.

iCloud and the System Photo Library

You can use iCloud Photos, Shared Albums, and My Photo Stream only with the System Photo Library. If you choose a different library in Photos without designating it as the System Photo Library, the iCloud tab in Photos preferences is disabled:

If you designate a new library as the System Photo Library and then turn on iCloud Photos, the photos and videos in the new library will merge with those already in your iCloud Photos. If you want to keep the contents of your photo libraries separate, don’t turn on iCloud Photos for more than one library in Photos.

To combine Photos libraries, open the source library and export the photos and videos that you want to keep. Then open the destination library (the one that you want to use as your main library) and import the photos and videos.

Choose a photo library

Here's how to open one of the multiple photo libraries that you might have on your Mac or on a connected external drive:

Press and hold the Option key as you open the Photos app.
Select the library that you want to open, then click Choose Library.
Photos uses this library until you open a different one using the same steps.

Export photos and videos from the source library

Open the source library, then decide whether you want to export your files as unmodified originals or edited versions:

Edited versions

Edited versions retain their edits when you import them. The advantage is that you don't have to recreate any edits; however, these files become the destination library's originals. This means that you can't revert to their earliest pre-import version.

Unmodified originals

Unmodified originals don't show any edits that you made when they were in the source library. This gives you flexibility for later editing, but recreating your edits might take substantial work.

How to export edited versions

In the Photos app, select the photos and videos that you want to export. To select multiple items, press and hold the Command key while you click. To select a group of items in order, click the first one, then press and hold the Shift key while you click the last one. This selects all of the items between the two that you clicked. To select all of the items in your library, go to the Library tab > All Photos, select an item, then click Command-A.
Choose File > Export > Export [number].
An export dialog appears.
- In the Photos section, set Photo Kind to JPEG, TIFF, or PNG. JPEG recompresses your photos, which may result in smaller file sizes. TIFF and PNG files are higher fidelity and may result in larger file sizes.
- In the Videos section, choose a Movie Quality setting. This section appears only if your selected items include videos.
- In the Info section, select the checkboxes if you want to preserve metadata and location data in the exported files.
- In the File Naming section, set File Name to Use File Name and set Subfolder Format to either Moment Name or None. Moment Name creates a subfolder for each Moment that's represented in your selected items. This is useful if you'd like to create an Album in the destination library for each Moment. The None option exports all of the files directly into the destination folder.
Click Export. A Finder dialog appears.
Navigate to the location where you want to save the files, such as the Desktop or an external drive. Click New Folder if you create a new folder for your exported items.
Click Export.

How to export unmodified originals

In the Photos app, then select the photos and videos that you want to export. To select multiple items, press and hold the Command key while you click. To select a group of items in order, click the first one, then press and hold the Shift key while you click the last one. This selects all of the items between the two that you clicked. To select all of the items in your library, go to the Library tab > All Photos, select an item, then click Command-A.
Choose File > Export > Export Unmodified Original.
An export dialog appears.
- If your photos include IPTC metadata (such as titles or keywords) that you want to keep, select the Export IPTC as XMP checkbox.
- Leave the File Name setting on Use File Name.
- Next to Subfolder Format, choose Moment Name or None. Moment Name creates a subfolder for each Moment that's represented in your selected items. This is useful if you'd like to create an Album in the destination library for each Moment. The None option exports all of the files directly into the destination folder.
Click Export. A Finder dialog appears.
Navigate to the location where you want to save the files, such as the Desktop or an external drive. Click New Folder if you create a new folder for your exported items.
Click Export Originals.

How to export your edited photos

You can create a Smart Album to quickly identify your edited photos and export them.

Mac photos how to open two libraries simultaneously using

To create a Smart Album for edited images:

Choose File > New Smart Album.
Choose Photo is [edited] from the Smart Album Criteria. The new Smart Album will appear at the top of the Albums list.
Open the album and press the Command key-A to select all, then choose File > Export.

How to import photos and videos into the destination library

In the Photos app, open the destination library and choose File > Import.
In the Import window, navigate to the Finder folder that contains the photos and videos that you want to import.
Select the photos and videos to import, then choose Review for Import.
Click Import All New Photos.

The photos in the folder are sorted based on their creation dates and locations. The videos are sorted based on the date you exported them from the source librar

Mac Photos How To Open Two Libraries Simultaneously