Police claimed that Sankaranarayanan confessed that he threw acid at the girls, both aged 17 and students of a college at Tirumangalam on September 12.
They said Sankaranarayanan appeared to be suffering from some "psychiatric problem" and further interrogation was on.
Meanwhile, the two girls, being treated at the Government Rajaji Hospital here, were recuperating and the injuries had almost healed, police said.
The girls, belonging to Chinnapoolampatti village near Thirumangalam, on their way to a bus stand after classes when the man threw acid on them and fled the scene.