“Offenders Clive Derby-Lewis and Janus Walusz were attacked by one offender with a kitchen table spoon,” said correctional services spokesman Manelisi Wolela in a statement.
“They both suffered some lacerations in their heads and hands at the Kgosi Mampuru Correctional Centre.”
They were treated and stitched by the prison nurses.
Police and correctional service officers were called in and the attacker was separated from them.
Derby-Lewis and Walusz told officials that the matter was “nothing serious”.
Officials continue to monitor the situation and the men may be sent for further observation.
Police were investigating the attack.
Derby-Lewis, now 78, was sentenced to 25-years behind bars for his role in Hani’s killing. He has already served almost 20 years.
Hani was shot dead in the driveway of his home in 1993. He was the secretary general of the SA Communist Party at the time.
During the trial, it emerged that Derby-Lewis arranged a firearm for hit-man Walus, 61, who is serving the same prison term.
Both men were initially sentenced to death, but this was commuted to life imprisonment when the death penalty was abolished.
Derby-Lewis, who was suffering from cancer, has twice been denied medical parole.
He first made an application in 2012, and another in 2011.