It is thought the driver of the second vehicle could hold key information as to what caused the horrific crash.
Four of the men arrested on suspicion of causing death by dangerous driving remain in custody for questioning, while a 23-year old man is assisting police with their enquiries, but is no longer police custody.
Sergeant Lee Westhead said: "This is a heartbreaking incident which has now taken the lives of two young girls and all of our thoughts remain with their families, who are understandably devastated.
"We have made significant progress with our investigation since the collision happened and I would like to thank everyone who has contacted police with information so far.
"The community are clearly as committed as we are to establishing exactly what happened to Helina and Zaneta and the support they have provided has been vital to the investigation."
Helina's mother Sylva and father Robert went to the scene in Oldham, Greater Manchester, but she was declared dead by medics as her mother stroked her hair.
On New Year's Day local time, the family of seven sisters and one brother, who came to England from the Czech Republic eight years ago, sat in tears mourning Helina.
Helina's sister Sylva, 19, told how the family rushed to the scene 100m from their home seconds after the crash.
She said: "She did not breathe. Mum was touching her hair."
The driver of the car did not stop and a police hunt for him was launched.
The girls were with a larger group of family and friends who had already crossed the road, one of the main routes in and out of Oldham town centre, and the larger group was waiting on the opposite pavement.
The two cousins then started to cross the road outside the shop.
Kotlarova said: "But as they came to the other side of the road the car ran them over. They were holding hands."
Witnesses described the car as driving fast and "flying away" after the collision. Seconds later, friends ran to Helina's home to alert the family.