You can have one device turn on when another device turns on, and turn off when that device turns off, by using the “mirror“ feature of smart lighting.
You can also keep the devices in sync if you want by creating two of these automations, One with switch A as the trigger and switch B as the target and another with switch B as the trigger and switch A as the target.
( there are a few switches which get confused by this and go into a loop if you use two automations with mirroring, but most switches can handle it.)
Here are the steps.
- choose the three horizontal line menu icon in the upper left of the screen, not the + in the upper right.
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choose smartapps from the pop-up menu.
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you should now see “smartlighting“ on the list of smart apps for your account. If you do not see it, you will need to contact support. Be aware that this smartapp is not available in all regions, although support may still be able to add it for you. Once you have it, select it.
- you will now see a list of any smartlights Automations you have already created, plus the option to create a new one.
- Tap on the line to select the device to be controlled, the target. In a virtual three-way, this is normally the master switch.
- Now you will see a list of all your devices. Just select one or more that you want as targets for this automation. make sure you click “done” at the bottom of the screen once you have selected all the ones you want.
- You will return to the main screen for creating the new automation and you should see the names of the target Switches. Now tap on the line where you specify what you want the target to do.
- This will take you to a screen where you can choose from turn on, turn off, or “mirror.“ Mirror means you want to target to turn on when the trigger turns on and you want to target to turn off when the trigger turns off. So for a virtual three-way, choose “mirror.“ then tap done at the bottom of the page.
- Again you will return to the main screen for creating the new automation. You should still see the names of your target devices and you should now see “mirror behavior“ filled in. Now tap on the line where you specify the trigger device, The switch to be mirrored. For a virtual three-way, This will normally be the accessory switch.
- This will now display a list of available switches. Choose the one you want to use for the trigger and then tap done at the bottom of the page.
- Again, you will return to the main page for creating an automation. Check to make sure that all the entries are correct, then tap “next“ at the bottom of the page.
- Finally, you can change the name of the automation if you want and save it.
That does it. Now you have an automation in smart lighting that will cause one device to mirror the behavior of another device. 
