Duotone balls are a sex toy. They resemble Ben Wa balls except that they are connected together by a cord and include a vibrating feature. The vibration may be electric and possibly remote controlled, or it may be generated by the movement of small weights inside the balls. Duotone balls are encased in plastic or silicone. They are designed to help strengthen the pelvic floor muscles. Many women also derive pleasure simply from wearing them, or even using them during sexual intercourse.
They should never be inserted into the anus due to the risk that they get so far in that they must be removed medically.