The trial would be the second Rwandan genocide trial in France, following former presidential guard Pascal Simbikangwa’s sentencing to 25 years in jail in March this year.
For the first time victims or relations of victims will appear in court, since nine have joined the case against the two men.
Ngenzi and Barahira were burgomasters (mayors) of Karabondo in 1977-1986 and 1987-1994 respectively and are accused of using their influence to lead local people to massacre their Tutsi neighbours at a meeting in April 1994 at the local football stadium.
Hundreds of people, mainly Tutsi, had taken refuge at the church and were slaughtered by the crowd.
Witnesses say that Ngenzi and Barahira played an active part in the massacre and that Ngenzi went on to hunt out and kill others who had fled to a health centre and an anti-illiteracy centre.
He claims that he was unable to stop the murderous frenzy.
Ngenzi was caught on the French Indian Ocean island of Mayotte in 2004 after applying for political asylum under a false identity.
Barahira was arrested in the south-western French city of Toulouse, where he was living, in 2013.
More than 20 investigations of Rwandans suspected of crimes against humanity are currently underway but France has never allowed the extradition of any suspects to Rwanda.