The iPhone Duo has no Action Button, but you can get close to the same functionality by swapping one of your lock screen icons for a control or Shortcut. Press and hold the lock screen, tap Customize, remove the Camera icon in the bottom right, and add a control of your choice in its place. A single tap then performs that action, whether it's turning on a Focus Mode or running a Shortcut.
The replacement won't match the original perfectly. The real Action Button has a quirk where pressing it does nothing except display a notice telling you to press longer, and a Shortcut or control can't reproduce that. Otherwise, the workaround covers nearly everything the Action Button did.
You can also choose the bottom left icon instead of the right one. The right icon defaults to the Camera, which remains accessible by swiping the lock screen or pressing the Camera Control button, so it is usually the most expendable.
The most flexible option is to have the button run a Shortcut. One Shortcut can turn on the torch, switch on a Focus Mode, and open an app, or it can act as a menu for your other Shortcuts. A control shows its own icon, while a Shortcut shows only the generic Shortcut icon.
Source: AppleInsider · Summarized by HeadlinesBriefing