If you’ve managed to obtain an N95 or an actual surgical mask (not the “medical looking” masks you can buy at the store), then putting another layer over the top probably doesn’t do much good. These health care masks are made of tightly woven materials specifically designed to prevent viral particles from penetrating, so they probably don’t need any extra help.
But if you’re using a basic cotton mask, wearing a second mask might indeed trap more viral particles in your breath when you exhale and might also provide better protection against inhaling the virus from the air. Recent research conducted on individual masks indicates that a two-layer cloth face mask performed better than a single-layer mask. So does that mean that the extra layers you’d get from double-masking would offer even more protection? We don’t have scientific evidence about that yet, but until we do, it probably doesn’t hurt to double-mask, especially if you find yourself in a higher-risk situation, like living with someone who’s sick with COVID-19.
That said, don’t develop a false sense of security that double-masking means you don’t also have to take other public health precautions. You still need to maintain physical distance from other people. And wash your hands.
Also, no matter how many layers of fabric your mask contains, make sure it fits well and hugs your face without gaps. Gaps can allow germs to escape -- or to get in and make you sick.
Lastly, remember that the best mask is the one you actually wear consistently. If double-masking makes you uncomfortable and tempts you to pull the mask down under your nose (a no-no!) or ditch it altogether, then just stick with your usual mask.