Police stopped the women after they drove into Saudi Arabia from the United Arab Emirates on December 1.
Loujain al-Hathloul, 25, had been planning to defy the kingdom's ban on women driving by crossing into her country from neighboring UAE, where she has a driver's license.
An activist says Saudi border guards confiscated her passport, forcing her to wait at the crossing for nearly 24 hours.
The activist and relatives say that when Maysa al-Amoudi, 33, arrived at the border to deliver food, water and a blanket to al-Hathloul, she too was subsequently arrested for driving. All spoke anonymously for fear of government reprisal.