Does the Bird 5014 Directional Power Sensor Require Use of Both Element Slots?

No, you do not need to make use of both element slots. If you want to just measure forward power, select the element that best meets your frequency/power requirements and seat it in the appropriate slot. Keep the dust plug covering the remaining slot to ensure nothing falls into the sensor body while power is on. 

If you want to just measure reflected power, again, select the element that best meets your frequency/power requirements (generally 10x less than the anticipated forward power) and seat it in the appropriate slot. Keep the dust plug covering the remaining slot to ensure nothing falls into the sensor body while power is on.