Why Nirbhaya's convict can't be hanged separately? 

In 1975, 3 accused were convicted and sentenced to death in a murder case by pilibhit court and was affirmed by Allahabad High Court. C1's SLP was dismissed by the Supreme Court and he was hanged in 1981. C2's SLP was allowed partly and his death sentence was commuted to life imprisonment by Supreme Court in 1978. C3's SLP, Review was rejected and President also rejected in mercy petition. C3 challenged the president order, whereby court looking at facts and circumstances of the case asked president to reconsider his decision on the ground that

"We have seen the facts from which it is clear that no distinction at all can be made between the part played by C2 on the one hand and the C3 on the other. Since C2's death sentence was commuted by this Court, it would be unjust to confirm the death sentence imposed upon the petitioner"

This became a correctional point, as court directed that the Jail Superintendent should ascertain  personally whether  the sentence  of death  imposed  upon  any of  the co-accused  of the prisoner who was  due to  be hanged had been commuted. If it had been commuted,  the Superintendent should apprise the superior authorities  of  the  matter,  who  must take  prompt steps  for brining the same to the notice of the courts. 

The Court decided that, the Considering that  the responsibility,  involvement and complicity of  the  petitioner in  the commission  of the offence is  the same  as that  of the  other co-accused,  it would be  sheer travesty  of justice if for the same offence the petitioner had to pay the extreme penalty of death while the death  sentence imposed  on the co-accused, for the very same offence  had been commuted to one of life imprisonment.

"Had the petitioner mentioned  either in  the special  leave

petition  or  review  petition or  mercy  petition  to the

President this all important  and significant fact,  death

sentence imposed on him would have been commuted."

This became a guideline lights for the hanging ever since in all cases where more than one convicts were on the waitlist to the gallows. Delhi Prison rules inserted a note to Rule 854 which says that if sentence has passed to more than one person in same case and if the appeal to higher court or an SLP in Supreme Court is lodged by or on behalf of only one and not all of them, the execution shall be postponed for all such persons. 

However, Ministry of Home affairs has lodged a Writ Crl in Supreme Court asking for the directions that the death convicts be given 1 week to file review, and from then 1 week to file curative and from then 1 week to file mercy petition and another one week to challenge president's order on mercy petition. 

The issue of Section 39A is a  complex scrutiny and whether state should have the power to take life is empirically and philosophically debatable, delaying the process of justice makes the administrative apparatus questionable. Tom tyler a Yale psychologist and law professor argues and argues well that, people follow for they see legitimacy in the authority which enforces law. if they see administration of justice being halted they start questioning that authority for the reason of it's efficacy. There is a fine balance to be maintained between rights of convicts and people's perception in cases of such outrage. People cannot be give a perception that justice system can be take to a ride. 

Case sited : Harbans Singh Vs State of U.P 1982 SCR (3) 235