According to Israeli police the incident took place when the car was passing near the illegal Israeli settlement of Kfar Oranim after nightfall.
Both Israelis were seriously wounded and were taken to hospital for treatment.
Aljazeera reporter Walid al-Umari said the attack was carried out by two unknown armed men.
The target was an Israeli patrol guarding a group of settlements in Belein west of Ram Allah, he added.
Israeli forces cordoned off the area searching for the assailants.
Truce in jeopardy
There were no immediate claims of responsibility for the attack, the second of its kind since Palestinian factions agreed to abide by an informal ceasefire with the Israelis.
The truce was agreed to at last month's Sharm al-Shaikh summit of Palestinian President Mahmud Abbas and Israeli Prime Minister Ariel Sharon.
The incident followed last Friday's bomb attack on a Tel Aviv nightclub, which killed five Israelis, most of whom were soldiers of an elite army unit who were there for a celebration.
The frequency of shooting incidents in the West Bank has dropped considerably in recent weeks, but the fresh outbreak of violence may puncture the ceasefire.