The 18-year-olds were attacked on Granby Row, in Manchester city centre, at about 02:15 BST.
Police said the offenders, believed to be in their 20s, approached the victims on Chorlton Street , walked with them and then assaulted them.
Police said two passers-by intervened and the attackers ran off. Detectives are urging witnesses to come forward.
Det Sgt Clare Smith, of Greater Manchester Police, said the victims were "understandably left extremely distressed" by the "disturbing" attack.
"This happened following a busy Saturday night in Manchester and it is possible that there could have been a number of people in the nearby area at the time that may hold key information," she said.
"I would like to pay tribute to the two men who spotted what was happening and quickly intervened."
A woman has been raped and her friend sexually assaulted by two men, during a night out.