The 20-year-old victim opened the door of her home in the town of Hilden early on Saturday when one of the suspects doused her with sulphuric acid from a tin can.
The victim immediately felt strong burns in her face and called rescue services who identified the liquid as an acid.
The woman was taken to a hospital. Her grandmother was also slightly hurt in the attack.
One of the suspects, a 22-year-old man of Turkish origin, was arrested and remanded in custody on Saturday.
He told investigators he had asked an acquaintance to carry out the attack, police and the public prosecutor's office in the state capital of Duesseldorf said in a joint statement.
The other suspect, who is aged 18 and thought to have carried out the attack, turned himself in to the police on Sunday.
"The attack was committed with sulphuric acid which the assailants bought in a pharmacy," authorities said on Sunday.
The victim had reported the suspected attacker three times to the police for threatening her and using physical violence.
Authorities said the young man had been ordered to stay away from her.