It is the fourth suicide bombing this month in Maiduguri, which is the birthplace of the Boko Haram Islamic extremist group.
Fishmonger Idi Idrisa said one teenager exploded as she approached the mosque crowded with people from the nearby Baga Road fish market, performing afternoon prayers during the holy month of Ramadan.
The second teen appeared to run away and blew up further away, killing only herself, he said.
Civilian defense fighter, Sama Ila Abu, said he counted at least 30 corpses as he helped to collect the dead. Both men said there were many injured being sent to the hospitals.
More From This Section
A military bomb disposal expert has told the AP that most bombs carried by girls and women have remote detonation devices, meaning the carrier cannot control the explosion.
Boko Haram has stepped up attacks since Nigeria's new President Muhammadu Buhari announced the military command center is moving from the capital Abuja to Maiduguri in Borno State.
The attacks come as Nigeria and its neighbors are preparing to strengthen a multinational army that this year drove Boko Haram out of the towns and villages where it had set up a so-called Islamic caliphate.
On Thursday, a group of the extremists attacked two towns in neighboring Niger, killing at least 40 people, the government said.
In its first attack on Chad, suicide bombers a week ago simultaneously attacked two buildings including the national police academy in N'Djamena, the Chadian capital, killing at least 33 people.