A case of attempt to murder has been registered against main accused Aamir and Mudassar, reported the News International.
Sarabjit suffered critical head injuries in the assault Friday.
The assault on Sarabjit comes a few months after the death of Indian prisoner Chamel Singh in the same Kot Lakhpat jail after he was allegedly assaulted by jail staff.
The First Information Report (FIR) says that the two prisoners attacked Sarabjit Singh soon after the gate of Barrack No.7 was opened.
The media report said Sarabjit was in jail for the last 21 years, Mudassar since 2005 and Amir since 2009.
A source in the Punjab Prisons Department told The News that Sarabjit was lodged in Block No.7 and he was taken out in the fenced courtyard at around 5 p.m. for a walk.
It was then that two other prisoners locked up in Block No.7-A and 7-B attacked Singh with a sharp-edged weapon.
The source said it was not yet clear how and from where did the attackers bring the weapon. He added the courtyard was also fenced and it was difficult to attack somebody there.
Sarabjit Singh has been on a death row in Pakistan since 1990 after being convicted by Pakistani courts for bomb blasts in Lahore and Multan cities which left 14 people dead.
Sarabjit's family in India claims that he had inadvertently crossed into Pakistan in August 1990 in an inebriated state and was arrested there.
But police in Pakistan claimed Sarabjit Singh, who is known as Manjit Singh in India, was involved in acts of terrorism.