Fiber is the best thing for diarrhea in rabbits, so try and encourage both to eat hay. For the rabbit that doesn't eat hay, get the freshest, greenest hay you can find and chop it up with scissors to make it easier to eat. In addition, source some rabbit probiotics, which can help recolonize the gut with the bacteria that help digestion. Be sure to keep their bottoms clean so they don't attract flies.