On an LT1 long style thermostat, should the bottom spring open at the same temperature as the top spring? What exactly is the purpose of the bottom spring?

 

As the thermostat opens, the bottom part elongates and seals a port in the pump. That allows the coolant to properly flow back and forth from the engine to the radiator for cooling. When the thermostat is closed the open port allows coolant to circulate more in the engine and pump.