But Cedric Roberts and Brendon Walters deny carrying out the brutal assault, with Roberts' barrister suggesting other potential suspects like the victim's cellmate might be to blame.

A hearing expected to last four days began in the ACT Magistrates Court on Friday, with Roberts and Walters each pleading not guilty to a charge of assault occasioning actual bodily harm.

In his opening address, prosecutor James Melloy told the court both defendants and the victim were locked up in the same unit at the Alexander Maconochie Centre at the time in question.

Mr Melloy said that about 5.10pm on December 16, 2018, Roberts and Walters entered the victim's cell and "violently assaulted him while inside".

A guard found the victim unconscious a few minutes later, with his cellmate rendering first aid.

Mr Melloy said he would rely on evidence including CCTV showing the defendants heading in and out of the cell at the relevant time, as well as DNA samples and the testimony of the victim's cellmate, Rhys Dugdale.

Mr Melloy said the victim had no memory of the incident, likely because of the head injuries he had sustained.

However, the man did recall having had an argument with Walters earlier the same day, which Mr Melloy said had provided a motive for an attack on him.

Mr Melloy also said Dugdale had described being threatened with a blade during the incident, and CCTV showed Walters throwing something in a bin soon after leaving the victim's cell.

But Roberts' barrister Jason Moffett, in his opening statement, suggested Dugdale may in fact have inflicted the victim's injuries.

He said he anticipated that Special Magistrate Jane Campbell would not be able to exclude that possibility, and would therefore have to find his client not guilty.

Walters' lawyer, Andrew Byrnes, noted that "a number of other suspects" could be seen on CCTV going into the relevant cell.

The court is set to hear from 15 prosecution witnesses, the first of whom was Canberra Hospital director of clinical forensic medical services Vanita Parekh.

Dr Parekh said the victim had been admitted to hospital on the day in question with a swollen face and upper lip, a bleeding scalp, bruising to the chest and left shoulder, and post-concussion syndrome.

She also told the court the victim had blood in his mouth and "wasn't able to manage his own airway", meaning he had to be intubated and placed in a coma.

Dr Parekh said she ultimately formed the opinion that the victim "sustained clinically significant injuries as the result of ... blunt force trauma to his head, face and chest".

She said these had been caused by multiple blows, possibly kicks and punches, while the victim's head had also likely hit "static surfaces" like a wall, bedframe or floor.

The victim spent parts of his hospital stay in the emergency department and intensive care unit.

The court also heard on Friday from a number of prisoners who were in the vicinity of the victim's cell at the relevant time.

One of them, Jacob Fort, remonstrated with Mr Melloy when he was asked in various ways about his recollections of the day in question.

"I've already said that I don't recall anything from this time, so what else do you f---en want?" he asked.

The hearing continues.

Our journalists work hard to provide local, up-to-date news to the community. This is how you can continue to access our trusted content: