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The teen, who was transgender, was stabbed 28 times in Culcheth Linear Park, Warrington on 11 February.
Girl X and boy Y, both 16, cannot be named because of their age.
Each had a fascination with violence and torture, a jury at Manchester Crown Court heard.
They both blamed each other for the murder and neither took responsibility for it.
Deliberations lasted for four hours and 40 minutes, with neither teen showing any signs of emotion as the verdicts were read out.
Sentencing is due to take place next month, though the judge told girl X and boy Y that they would each be serving life sentences – the only thing to be determined is the minimum terms they will face.
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During the trial, the court was shown a “murder plan” which detailed how to kill Brianna.
It also included a “kill list” of four other children.
Ursula Doyle, Deputy Chief Crown Prosecutor, said the two teens had been “a deadly influence on each other”.
“The pages and pages of WhatsApp messages between the two, planning and plotting to kill people, talking of murder, torture and cruelty were very difficult to read,” she said outside the hearing.
“The loss of her young life has left a massive hole in our family”
Brianna’s family described her as a “larger-than-life character who would leave a lasting impression on all that met her”.
“The loss of her young life has left a massive hole in our family, and we know that the teachers and her friends who were involved in her life will feel the same,” they said shortly after her passing.
“We would like to thank everyone for their kind words and support during this extremely difficult time.”