Here are two mask combos you shouldn't use according to BestLife.com . . .
1. Don't wear a second mask over an N95 mask, or KN95 mask. N95s are designed to fit tight and make a seal. And wearing a second mask on top of it could make it fit worse. Plus, it can be a little hard to breathe.
2. Don't use two disposable masks on top of each other. We're talking about the blue, surgical ones a lot of people wear.
They're NOT designed to fit tight and make a seal. But wearing a second one can mess with the fit and prevent you from breathing through the fabric. So more air comes in through the sides.
You CAN wear two cloth masks though. Or a disposable mask with a cloth mask, because they tend to fit better that way. The CDC recommends putting the disposable one on first, and then a cloth mask on top of it.