50:50 radio
The radio is sold as a pair, however, only one radio can be controlled at a time.
The person in control can tune the radio by twisting the antenna while the other radio's antenna will be down. The person with no control over the radio can listen to what the other radio is tuned to and does not gain control until the other radio is switched off.
It may seem like luck at first, but once either user discovers each other's routines, patterns are then revealed.

What effects will the two radios have on the users?
Since the radios are permanently linked, how does the relationship between the two users influence the way the product is treated?
What if the radio was adopted on the street by a stranger?