Can I stack two ridge vents on top of each other to double or increase the Net Free Area airflow?

Airflow performance is not doubled after stacking two ridge vents on top of each other; and it will look unsightly.

Our testing showed that airflow performance was not doubled after stacking two ridge vents. Furthermore, in the eyes of many homeowners, any airflow gain would be negated by the unattractive appearance of such an application.