Did they really? We all know more layers is better for this kind of diffusion filter. However, the other key variables are fit and leakage, where double masking

Did they really? We all know more layers is better for this kind of diffusion filter. However, the other key variables are fit and leakage, where double masking tends to not help at all with respirators, but may with very soft masks. We don't have any data that it does, though. Usually people use sub par (cloth, surgical), badly fitted (loose rubber bands, misfitting nasal) masks or wear them wrong - the classic being a mask not covering the nose.