The thing is...sometimes they CAN be kept in pairs. But usually they can't. One female asserts herself as the dominant of the pair, and then, to keep her position as dominant, she is aggressive with the other female. This can result in injuries, illness, and death.
Signs of stress (especially in females) is paling of color overall and then horizontal stripes on the body. Vertical stripes are a sign of aggression and the desire for dominance, and, in the case of breeders, readiness to breed. Between two females, it is invariably the former. Between a male and a female, it can be either former or latter.