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_v1.1.yaml

blueprint:
  name: Link On/Off State of Multiple Devices
  description: |
    ## Link On/Off State of Multiple Devices v1.3
    1) It triggers not only on a state change, but also every minute, to eliminate a problem with very fast changes on/off with last one not triggering anything and entities become unsincronized.
    2) It does not trigger when initial entity state is unavailable or unknown. It solves the problem with a ton of zigbee devices firstly sending this status after HA reboot, then sending their real one, trigerring all automations based on the v1 template.
    3) Fixed log errors

    ### Credits
    * [@luma](https://community.home-assistant.io/u/luma) for [v1 template](https://community.home-assistant.io/t/link-on-off-state-of-multiple-devices/460310)
    * [@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: restart
max_exceeded: silent

variables:
  linked_entities: !input 'linked_entities'

trigger:
- platform: state
  entity_id: !input 'linked_entities'
- platform: time_pattern
  minutes: /1

condition:
- condition: template
  value_template: '{{ trigger.from_state.state != "unavailable" }}'
- condition: template
  value_template: '{{ trigger.from_state.state != "unknown" }}'
- 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 }}'

Instead of this:

- condition: template
  value_template: '{{ trigger.from_state.state != "unavailable" }}'
- condition: template
  value_template: '{{ trigger.from_state.state != "unknown" }}'

wouldn't it be better to do this?

- condition: template
  value_template: '{{ (trigger.from_state.state == "on") or (trigger.from_state.state == "off") }}'

There might be more states (other than unavailable/unknown) in the future that could cause errors, so instead of checking against all possible negatives, we should only validate allowed states/attributes?

This blueprint works great. It creates a great deal of log errors because to_state and from_state can't always be found. So, I am checking for their availability in my fork: https://gist.github.com/shageman/dbc21d5d50095053c6d0616076a0d76b