The brown "probe" on the heat cable is not a sensor. It is the end of the heat cable, where they terminate the conductors. Insulation only. It doesn't heat up either. You tape it wherever out of the way.
The gray Ranco probe is the heat sensor to tell the Ranco what the temp is. You place that where you want to control the temp. Some people place it on the heated area of the rack, some in the middle, some in the cool area.
With ON/OFF controllers like Rancos, I place the probe near the heater but not on it. I have a good reason for this, but I'll spare you the details.
So, place the probe somewhere, set the Ranco for the max temp you want to see where the probe is. Set the differential (diff between the OFF temp and back ON temp) somewhere between 2-4 degrees usually.
-----
Mark