Both wearing masks, obviously. But that isn't what the title says. It is saying that a mask with a valve doesn't slow the spread of covid. Obviously, one of us

Both wearing masks, obviously. But that isn't what the title says. It is saying that a mask with a valve doesn't slow the spread of covid. Obviously, one of us wearing a mask is better than neither of us wearing a mask. I think maybe the implication is that if the person wearing the mask already has covid, it isn't going to slow the spread to others. Which makes sense. But if you at least wear a valved mask, you will be less likely to catch covid in the first place.