The reason why the double filters reach a higher terminal velocity than a single one is the same as the reason that a 120 kg parachutist falls faster (terminal speed) than does a 60 kg parachutist. (Identical parachutes assumed).
At terminal speed, the air drag force (proportional to the square of the velocity, but does not depend on mass) equals the force of gravity ( equal to mg and does not depend on velocity). Thus more mass means more gravitational force, which means more air drag needed, which means higher terminal speed.
