DUAL 3D printers have two filament sensors. One is in head 1 and the other in head 2. However, the detectors do not work independently but by an all or nothing system. In the case of DUAL, it is important that head 1 has a loaded filament because head 2 is dependent on the status of head 1. We say that head 2 is “slave” of head 1.
If you find that the filament sensor is out of adjustment, we have a topic dealing with this anomaly.