The official BNA news agency cited acting chief prosecutor Mohammed Salah as saying the two were found guilty of premeditated attempted murder and possessing a Kalashnikov assault rifle and ammunition.
BNA said the prosecution found that the "defendants had intended to kill a number of policemen" and "waited for the victims and shot at their vehicle".
Scores of Shiite Muslims have been tried over unrest in the Sunni-ruled kingdom that followed Arab Spring-inspired protests in mid-February 2011.
Last year the authorities increased the penalties for those convicted of violence, introducing the death sentence or life terms in the case of people being killed or wounded.
