Richard Barlow was convicted of eight offences after a trial and sentenced two days before his 40th birthday, at Nottingham Crown Court, on Wednesday.
The girls were both under 16, when Barlow, a music teacher, preyed upon them in their tent at the Pentrich Rock and Blues Festival.
The semi-professional guitarist also received a concurrent sentence for possessing indecent images of another girl.
Recorder Michael Auty QC told Barlow: “It seems that they did not take much of the cocaine. It had an immediate impact, particularly on one girl. It certainly stupefied them for moments.
"You undressed them both and penetrated the vagina of each of them. I regard what you did as having elements of real wickedness to it."
He ruled that Barlow poses a significant risk of serious harm to adolescent girls and imposed an 18-year extended sentence, with a three-year licence period.
Barlow, formerly of St Edmund’s Avenue, Mansfield Woodhouse, was placed on the sexual offences register for the rest of his life, and made the subject of sexual harm prevention order without limit.
One of Barlow’s victims spoke of suffering depression, suicidal thoughts, panic attacks and ill health.
“I still have bad days and the constant guilt of not speaking up sooner could have stopped it happening to someone else,” she said. “I hope others are now protected.”
The other said: “At first I blamed myself. I started getting flashbacks. I would lie awake at night replaying what happened and thinking of ways to prevent it.”
Sarah Phelan, mitigating, said: "Custody is going to be hard. His mental health is very poor at the moment.”
The judge told the two victims that attended court: "I have been involved in cases like this for 35 years. It breaks my heart to see young people have to come to court and speak of these things.
"That you found the courage to do so, does you great credit. You should be very proud of yourselves. I hope sincerely that you will be able to come to terms with what has happened.
"You are two remarkable young women with bright futures and I wish you all the very best."
Speaking after the hearing, Detective Constable Richard Howe, of Nottinghamshire Police, said: “Barlow exploited girls who were less than half his age. As we revealed during our investigation, he knew full-well that what he was doing was wrong but carried on with his offending regardless.
“His victims, who have shown enormous courage and dignity during this process, must live every day with the memory not only of what Barlow did to them, but also of the crown court trial he forced them to sit through as he attempted to defend the indefensible.
“I am pleased that he has finally been exposed for what he is and hope this case also serves as a reminder to other men who think they can get away with abusing children. All it takes is one call to the police for them to be exposed and shamed for what they are.”