Yolanda Molina Ugarte, the defendants' lawyer, said her clients would challenge the ruling at France's high court of appeal.
Molina has accused the French authorities of violating procedures to prolong the detention of the two men.
She says Paris is trying to "quench the thirst for repression" from Spanish authorities following a December 30 bomb attack.
The bombing in a car park at Madrid's Barajas airport was the first time that Eta had killed anyone in an attack since May 2003.
It brought efforts for a negotiated end to separatist violence to a halt.
For nearly four decades Eta has waged an armed campaign for an independent Basque homeland - comprising parts of northern Spain and southwestern France - that has claimed more than 800 lives.