The pair were jailed on Saturday night after appearing before judges who charged them with leading an organisation established to commit crimes linked to terrorism.
Other charges included receiving funding for terrorism and possession of weapons and counterfeit documents.
Two other alleged Eta members, French national Pantxo Flores and Spanish national Ramon Zagarzazu, were charged with criminal conspiracy in relation to a terrorist enterprise.
Sources in both the French and the Spanish police estimate that there are now less than 30 Eta members at liberty, with about 455 behind bars in both countries.
The only known leader still free and living underground is 64-year-old Josu Ternera.
Since 2011, when it declared an end to its armed struggle but did not surrender its weapons, Eta has been trying to negotiate the fate of its members in prison in exchange for its dissolution.