If you have the original model, that is a high temperature sensor that will shut of power to the original element once the cabinet gets too hot. The old cutoff temperature was 320°F. The later models; manufactured later than 2010 or there about would cut off around 300°F; but there is a 40°F, either way. So it could be higher or lower.
If you install separate wiring for the second element, if the high temperature sensor is tripped it will shut down the power to the original element, but the second element will continue to function. If you use piggyback wiring as TedEbear wired his, when the high temperature sensor shuts down both the original element, and the second element. Having both shut off could be a safety feature. But if the sensor does not reset, you don't have any heating element to cook with. If you wire the elements separately, the second element will continue to work. Either way you wire, the best safety feature to have is a PID, or some other temperature controller.