The same features that drive up the cost of a single element Thermal-Ribbon, exponentially increase the cost of a dual element part. Since materials are relatively insignificant in a Thermal-Ribbon (parts are usually small), there is not a lot of material cost savings for a dual element. Tight resistance and dimensional tolerances are the biggest factor. If just one element is out of tolerance, the entire unit has to be rejected. If precision measurements are of concern for you, consider having the parts calibrated at the actual temperature you are using the parts at or ask for measurement readings and calibrate your instruments to to the sensor. Both of these options improve performance of the system, while keeping costs down.