Medical experts expressed concern on Wednesday about the haste and motives behind the 52-hour surgery which lead to the death of the twins on Tuesday.
"There are troubling aspects about this case," Dr Kerridge, Associate Professor in Bioethics at Sydney University said.
"And one of them was the statement by one of the surgeons that they found it was more difficult than they had expected. To me that sets off a little bit of an alarm bell," he added.
The 29-year-old sisters Ladan and Laleh Bijani died within 90 minutes of each other after the separating of their heads caused massive blood loss.
Kerridge suggested doctors could have let the girls wait a year, talk to people who have not had the surgery or to people who felt it was wrong.
Twins joined at the head occur once in every two million live births. A separation operation had never been tried on adults.
Decision was right
Doctors who performed the operation defended their decision.
"I think that for those of us who were here over these last three days, for those of us who flew in from all over the world...the time and commitment is a convincing indication of their belief that the decision is correct," Dr Keith Goh, who led the team of 28 specialists and 100 assistants told a news conference.
Ben Carson, director of paediatric neurosurgery at Johns Hopkins University in Baltimore also said the operation was worth it "even recognising that the odds were not good. I think it was a worthy humanitarian effort."
But a neurosurgeon in Germany who declined to separate the twins when they were 14 said on Tuesday he was surprised the operation had even been attempted.
Madjid Samii, president of the International Neuroscience Institute in Hanover, turned down the twins' request after examinations showed the shared vein, that drained blood from their brains to their hearts,and which meant chances of their survival were almost nil.
Prayers were held on Wednesday for the twins whose bodies will be flown back to Iran from Singapore on Thursday.
The Islamic Religious Council of Singapore released a statement ahead of the service reflecting the emotions of many Muslims and non-Muslims in the city-state, which had been the twins' home since November last year as they prepared for the operation.
"Although they were aware that their surgery was always going to be a complicated one, they persevered. Indeed their enthusiasm was commendable," it said.
The twins were born to a poor farming family in southern Iran with the sides of their skulls joined and sharing a vital vein that drained blood from their individual brains.
Ladan wanted to pursue a career as a lawyer in her home town of Shiraz and Laleh wanted to be a journalist in Tehran.