Check out Serial Protocol
I have four Arduinos. Each has a different MY_NODE_IDs (I use 130, 131, 132, 133). Each has four sensors: moisture, humidity, temperature, and voltage each of which has a CHILD_ID: CHILD_ID_MOISTURE is 0 , CHILD_ID_HUMIDITY is 1, CHILD_ID_TEMPERATURE is 2, CHILD_ID_VOLTAGE is 3.
When an Arduino makes a measurement, it sends that measurement to a unique MQTT topic. In the case of the Arduino with MY_NODE_ID of 130 that is sending the moisture measurement, that MQTT topic is mysensors_out/130/0/1/0/35 and the message is [something like] 760 (35 is the value type)
Similarly, when a controller sends a command to a device (Arduino), the topic is unique.
If you have two actuators with the same MY_NODE_ID and CHILD_ID (MQTT topic) there would be confusion as to what you want each actuator to do. Likewise, if two sensors had the same MQTT topic, how would the controller know how to handle the data.