Last night's Intervention featured a set of anorexic twins. They've developed rules to always eat and burn off equal amounts of calories (limited to 300 per day), which entails making the same movements, and walking the same amount of steps.
Being fraternal twins, Julia is three inches taller than Sonia. Growing up, Julia weighed more than her sister, which helped contribute to eating disordered behavior. When Julia dropped to 86 pounds, Sonia felt like "the fat twin," and became competitive with her sister on how little the two could eat. As a way to control the competition, the girls decided to come up with their rules, and began mapping out what they would eat each day, so that it would remain equal. Their obsession caused them to drop out of college, and move in with their parents, where they sleep in the same bed and live their lives around their shared disorder.
At the intervention, the girls agreed to get help at a clinic, and seemed to be improving by the end of the episode.