2 discus only, is a bad number unless its a pair. Generally a group of about 6 or more helps to make them secure. They won't school with tetras really.
Do you age your water? Do you have chlorine or chloramines in your water? What product do you use to treat your water to rid of chlorine or chloramine?
Going dark and fast breathing is stress, could be from the water or an illness. Where did you get the fish? Did you quarantine properly, to include plants?