On heavy equipment or ag equipment that run a Donaldson setup, the inner element is generally a "safety" element primarily intended to prevent stray dust from getting into the intake tract during servicing of the main element, and are made of different material. Over-servicing air filters is a big no-no in that world, and that's why they have an air restriction indicator, or "filter minder". I don't recall seeing any with two pleated elements, though.