Changing alternate icon for iPad

Attempting to give a more re-useable (easy to copy/paste) version of this answer.

1) You need images of the following names and sizes added as regular .png files (no asset catalogues)

.../AppIcon-Dark/iPad-app.png
  pixelWidth: 76
.../AppIcon-Dark/[email protected]
  pixelWidth: 152
.../AppIcon-Dark/[email protected]
  pixelWidth: 167
.../AppIcon-Dark/[email protected]
  pixelWidth: 120
.../AppIcon-Dark/[email protected]
  pixelWidth: 180

you can then add/insert the following in your info.plist

<key>CFBundleIcons</key>
<dict>
    <key>CFBundleAlternateIcons</key>
    <dict>
        <key>AppIcon-Dark</key>
        <dict>
            <key>CFBundleIconFiles</key>
            <array>
                <string>iPhone-app</string>
            </array>
        </dict>
    </dict>
    <key>CFBundlePrimaryIcon</key>
    <dict>
        <key>CFBundleIconFiles</key>
        <array>
            <string>AppIcon</string>
        </array>
    </dict>
</dict>
<key>CFBundleIcons~ipad</key>
<dict>
    <key>CFBundleAlternateIcons</key>
    <dict>
        <key>AppIcon-Dark</key>
        <dict>
            <key>CFBundleIconFiles</key>
            <array>
                <string>iPad-app</string>
                <string>iPad-pro</string>
            </array>
        </dict>
    </dict>
    <key>CFBundlePrimaryIcon</key>
    <dict>
        <key>CFBundleIconFiles</key>
        <array>
            <string>AppIcon</string>
        </array>
    </dict>
</dict>

I then set my icon with the following

func updateIcon() {
    if #available(iOS 13.0, *) {
        let dark = window?.traitCollection.userInterfaceStyle == .dark

        let currentIconName = UIApplication.shared.alternateIconName
        let desiredIconName:String? = dark ? "AppIcon-Dark" : nil

        if currentIconName != desiredIconName {
            UIApplication.shared.setAlternateIconName(desiredIconName) {
                (error) in
                print("failed: \(String(describing: error))")
            }
        }
    }
}

Your info.plist is structured incorrectly.

You have:

- CFBundleIcons
  - CFBundleAlternateIcons
    - Icon Name
      - CFBundleIconFiles
      - CFBundleIconFiles~ipad

But it should be:

- CFBundleIcons
  - CFBundleAlternateIcons
    - Icon Name
      - CFBundleIconFiles
- CFBundleIcons~ipad
  - CFBundleAlternateIcons
    - Icon Name
     - CFBundleIconFiles

Basically, once you have it working with iPhone icons, CFBundleIcons, duplicate the entire tree as CFBundleIcons~ipad. The iPad files shouldn't be nested under CFBundleIcons at all.

You're mixing up CFBundleIcons~ipad and CFBundleIconFiles~ipad (which isn't a valid key).