This can be serious and difficult to treat, so don't ignore it. Mouth rot is not an accurate name for anything. It is an infection (stomatitis) that can spread to the jaw, bone, cheek, and eyes if not treated. Eric is right...if they both have it you need to check the nutrition of your gutloads for deficiencies. Use more citrus in your gutloads. Vit. C and the lower ph may help keep this from recurring. Also, get them out into unfiltered sunlight when you can. This can boost the immune response and general health.