Two sov’s are used in order to have redundant air supply arrangement to valve. Now consider following cases:
Both SOV A & B are Energized:
In this case air supply is through SOV A, since SOV B exhaust is blocked because vent port of SOV A is closed.
SOV A Failed and SOV B Energized:
Since SOV A is failed so vent/exhaust port of SOV A is now open, hence exhaust of SOV B will go via vent port of SOV A to SOV exhaust port.
SOV A Energized and SOV B Failed:
Now SOV B vent port/exhaust port is open and SOV A is in line, but at the time of SOV A deenergizing vent of SOV A will pass via exhaust port of SOV B to vent port of SOV A. Hence depresurizing the system.