This is a straight forward and native method that works locally (in case you also want to tie the master switch to other automations).
- In SmartThings mobile app, go to Automations / Add a SmartApp > More > Virtual Device Creator : create a new virtual switch/dimmer.
- Then, go to Add a SmartApp > Lights & Switches > Smart Lighting > New Lighting Automation.
- select devices you want to control
- under "What do you want to do?" select "Mirror Behavior"
- under "Mirror which switch" select the virtual switch that you created in the first step
- Now, the master switch can be added as a Tile.
- If you don't see the newly created virtual switch in your ActionTiles device inventory, in ActionTiles go to My Locations / <Location> and tap SYNCHRONIZE.
Alternatively, if you don't want to create a virtual switch, you may chose to designate one of the devices in the group as the master switch.
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