Use this Shelly smart relay script to sync the switch state of the relay with that of another device using MQTT.

ShellySyncWithPairedLight.js

function subscribeToStatusUpdates(topic) {
  MQTT.subscribe(topic , function(t, message) {
    if (typeof message === "undefined" || message === null) {
      print("Received undefined or null message - ignoring");
      return;
    }
  
    try {
      let payload = JSON.parse(message);
      if (payload.source === "button") {
        let desired = payload.output;
        let localStatus = Shelly.getComponentStatus("switch", 0);
        if (localStatus.output !== desired) {
          Shelly.call("Switch.Set", {id: 0, on: desired});
          print("Synced local relay to " + (desired ? "on" : "off"));
        }
      }
    } catch (e) {
      print("Error parsing payload: " + e);
    }
  });
  print("Subscribed to: " + topic);
}

function run(topic_key) {
  Shelly.call(
    "KVS.Get", 
    { key: topic_key },
    function (topic_value, error_code, error_message) {
      if (error_code === 0) {
        subscribeToStatusUpdates(topic_value.value)
      } else {
        print("Error fetching KVS: ", error_message);
      }
    }
  );
}

// Set the value of key "paired_light_status_topic" in KVS, e.g.
// "shelly-xyz-lights/status/switch:0". MQTT must be set up on
// this device and on the remote one. The remote device must
// publish general status updates via MQTT.
run("paired_light_status_topic");