 
					
						
					
				
				
					@Knightan
I believe that your problem may be related to this block of code. See my thoughts as comments....
 if (message.type == V_STATUS) {  //This is true for all messages received
   // Change relay state
    state_1 = message.getBool(); //This sets state for state_1 to the received value. 
    digitalWrite(LED_PIN_Channel_1, state_1?RELAY_1_ON:RELAY_1_OFF);
    // Store state in eeprom
    saveState(CHILD_ID_5, state_1);
    
     // Write some debug info
     Serial.print("Incoming change for sensor: STATE 1");
     Serial.print(message.sensor);
     Serial.print(", New status: ");
     Serial.println(message.getBool());
   } 
if (message.type == V_STATUS) {
   // Change relay state
    state_2 = message.getBool();  //This sets state for state_2 to the received value. 
    digitalWrite(LED_PIN_Channel_2, state_2?RELAY_2_ON:RELAY_2_OFF);
    // Store state in eeprom
    saveState(CHILD_ID_6, state_2);
    
     // Write some debug info
     Serial.print("Incoming change for sensor: STATE 2");
     Serial.print(message.sensor);
     Serial.print(", New status: ");
     Serial.println(message.getBool());
   } 
It seems to me that state_1 = message.getBool(); AND state_2 = message.getBool(). So they both get set to the incoming value.
I think you need to not set both to the same value and instead differentiate them to unique values. I could be wrong as I have not had chance to do anything related to mysensors for a long time now - I also don't use Domoticz.
Have a look at the code on this page, it might help you a lot....     https://www.mysensors.org/build/relay