Tests have found that doubling layers in DIY masks improves the mask’s filtration effectiveness. However, the increase in effectiveness is far from double. For example, a double-layer dish towel captured 14% more particles than a single layer as shown in the picture below. But the improvement was only 1% for a pillowcase and 2% for a T-shirt. Doubling will also reduce the breathability of the mask.
We have more data on the effect of doubling mask layers.