The analog and digital ports on the Control Hub and Expansion Hub combine two sensor inputs into one connector to save space. Single sensors plugged directly into these ports will work, but they will block access to the second input. The Sensor Splitter Cable will allow two sensors to be plugged into the same port as needed. 
Be sure to check that the pinout of your sensor is compatible with this cable as some sensor pinouts will require modification or splicing of the splitter cable to make it compatible.