Select multiple entities to link their on/off state. If any selected entity is turned on or off, the other selected entities will be sent a matching on or off command.

link_multiple_devices.yaml

blueprint:
  name: Link On/Off State of Multiple Devices
  description: |
    ## Link On/Off State of Multiple Devices v1.0.0

    Select multiple entities to link their on/off state.  If any selected entity is turned on or off, the other selected entities will be sent a matching on or off command.

    You can select any entity, but only entities supported by the `homeassistant.turn_on` or `homeassistant.turn_off` service calls will work.

    ### Requirements
    * All selected entities MUST suport `homeassistant.turn_on` and `homeassistant.turn_off` or errors will be logged and the blueprint will not work.
    * Requires Home Assistant 2022.5.0 or newer.
    
    ### Credits
    * [@adchevrier](https://community.home-assistant.io/u/adchevrier) for the [initial blueprint](https://community.home-assistant.io/t/synchronize-the-on-off-state-of-2-entities/259010)
    * [@hebus](https://community.home-assistant.io/u/hebus) for [this fantastic template](https://community.home-assistant.io/t/synchronize-the-on-off-state-of-2-entities/259010/38)
  domain: automation
  homeassistant:
    min_version: 2022.5.0
  input:
    linked_entities:
      name: Entities to link
      selector:
        entity:
          multiple: true

mode: queued
max_exceeded: silent

variables:
  linked_entities: !input 'linked_entities'

trigger:
- platform: state
  entity_id: !input 'linked_entities'

condition:
- condition: template
  value_template: '{{ trigger.to_state.state != trigger.from_state.state }}'
- condition: template
  value_template: '{{ (trigger.to_state.state == "on") or (trigger.to_state.state == "off") }}'
- condition: template
  value_template: '{{trigger.to_state.context.parent_id is none or (trigger.to_state.context.id != this.context.id and trigger.to_state.context.parent_id != this.context.id) }}'

action:
- service: homeassistant.turn_{{ trigger.to_state.state }}
  target:
    entity_id: '{{ expand(linked_entities) | selectattr("entity_id", "!=", trigger.entity_id) | map(attribute="entity_id") | list }}'

Thank you for sharing. However there is a bug. If you link up 2 devices and operate On/OFF on one device and switch to other, it might take a couple of tries to sync again.

@aderusha You may be interested in my tiny change that seems to fix the infinite on/off cycling if an on and off are received too close together. https://gist.github.com/dbrand666/b731672b2a282b9afd51a97e6e5c2b2b

I've also seen infinite cycling with sonoff linked switches. Thanks @dbrand666

@dbrand666 Your tiny change is about the mode? from queue to restart correct?

How is it possible to get a infinite loop on/off with this security '{{trigger.to_state.context.parent_id is none or (trigger.to_state.context.id != this.context.id and trigger.to_state.context.parent_id != this.context.id) }}' ?

I did an analysis here:
https://community.home-assistant.io/t/synchronize-the-on-off-state-of-2-entities/259010/81?u=dbrand666
I think I went a little overboard - assuming there are multiple controllers. In HA, there's only one queue we really need to worry about.

There is a bug in this blueprint. If you turn lights on and off in quick succession they desynchronize sometimes and keep turning on and off... Fast... This could wear out the switches if this happens randomly or at night where you might not notice. I guess some small delays could be introduced to double check the state?

After some tweaking it seems that the mode is the issue for me. Changing this to restart fixed the issue. Perhaps some improvements could be made here?