Police, who had been hunting the pair since the end of last week, seized them a few minutes apart in the southeastern port city, the police source said.
The police source said the two men appeared to have become radicalized during a term in prison. A search of a rented apartment was underway, Reuters reported.
France goes to the polls to choose a new president on April 23, with a second round of voting due on May 7.
France has been under a state of emergency, which has been extended several times, in the face of Islamist militant attacks in Paris and other parts of the country in which more than 230 people have been killed.