They should not be used anally because they can easily become irretrievable (except by a doctor - yikes!).
The term duotone balls is often used interchangeably with Ben Wa balls, but in the most traditional sense, Ben Wa balls are simply a set of weighted ball bearings.
More About Duotone Balls
Duotone balls come in a string of up to four balls connected together. They are inserted into the vagina and worn to exercise the Kegel muscles and increase arousal.