No experience with domoticz, but this may help nevertheless: https://github.com/rejoe2/MySensors-Dallas-Address-ChildID-Consistency
You will find some sketches for using more than one DS18B20 on one bus and some explanation also.
General remark: as long as you do not change your hardware setup, you should find the same point of measurement reported under a specific temp-Child-ID using the standard sketches. The sketches above go a little further wrt. this.