Cedric Roberts, 24, and Brendon Walters, 27, faced the ACT Magistrates Court on Thursday after they were found guilty of the vicious assault.

The men pleaded not guilty to assault occasioning bodily harm.

In her fact finding, Ms Campbell said the offenders entered the victim's cell in December 2018 while he was asleep before Walters kicked him in the head, causing his head to forcefully hit a wall.

Both men then pulled the victim, who said in an impact statement he thought the offenders were his mates, off his bed before kicking his body numerous times.

The violent attack, behind a closed door, lasted about 90 seconds, leaving the victim unconscious.

When jail staff discovered what had happened minutes later, they called for help and the victim was placed into an induced coma at the Canberra Hospital.

He was in intensive care for two nights with subsequent medical evidence showing he suffered post-concussion syndrome, which includes short-term memory loss, among other injuries.

Ms Campbell said she regarded "both offenders engaged in significant violence" and that the victim was defenceless because he was asleep.

"It was planned, it was designed to cause actual harm to the victim," she said.

"Other than the evidence of an earlier argument that morning regarding a tattoo device, there does not seem to be any other motivation for the offence."

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She said that while Walters delivered the kick that caused the victim's head to hit a wall, which possibly caused the post-concussion syndrome, she found both men "equally responsible for the entire assault".

Ms Campbell sentenced Roberts, who was already serving a jail term until May 2024, to another two years from May 2023, which includes other violent offending, including him using a fry pan to hit jail staff.

He had also spat at a staff and damaged his cell.

The court heard Roberts has spent most of his time in custody with Ms Campbell saying there was "a real risk of institutionalisation".

"Mr Roberts is still a young man. Of course he's an adult now and he is responsible for his own conduct," she said.

"However, I cannot ignore that his childhood experience has left him ill-equipped to rationally handle his frustrations, anger and resentment."

She sentenced Walters to 18 months' jail - from March 2021 until September 2022 - for the assault and suspended jail terms for unrelated offences that had occurred earlier while he was on parole.

During her sentencing, Ms Campbell also took into account the delays in laying charges without explanation and said the matters could have been dealt with at "a much earlier time".

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