The driver, aged just 15, died that evening, while the 13-year-old passenger died overnight.
Both were from a community of travellers living near Chiètres.
The accident happened at around 1730 on Thursday when the car left the road for reasons unknown and rolled down an embankment, police said.
It came to a stop upside-down, partly submerged in a canal.
Another man, also from the travellers’ community, alerted emergency services who took the two teenagers to hospital where they later died.