The General Medical Council is investigating two more doctors linked to the Hyponatraemia Inquiry.
The inquiry examined the deaths of five children in Northern Ireland.
Dr Heather Steen and Dr Robert Taylor were involved in the care of Claire Roberts who died at the Royal Belfast Hospital for Sick children in 1996.
In documents seen by the BBC, the General Medical Council said that in these cases it is in the public interest to waive the five-year rule.
The rule normally means a complaint against a doctor has to be made within five years of any incident.
The BBC has asked the Belfast Health Trust and the doctors for comment.
In January, an inquiry into the deaths of the five children in hospitals found that four were avoidable.
Hyponatraemia occurs when there is a shortage of sodium in the bloodstream.
Waiving the five-year rule is significant, as some other doctors named in the hyponatraemia report will not be investigated as they have either retired or the GMC set time has lapsed.
"Our fitness to practice rules impose a five-year limit period on complaints, which means that we cannot generally investigate complaints about events which took place over five years ago," the GMC said.
"However, we do have discretion to waive the five-year rule where we consider that it is in the public interest for us to investigate."
Alan Roberts, whose nine-year-old daughter, Claire, was one of the children who died, said he welcomed the development.
Speaking to the BBC, Mr Roberts said it was another stage in a very difficult journey.
"The GMC have identified and will now fully investigate the role of two senior doctors," he said.
"The GMC must also consider and review the actions and failures of all other doctors involved in our daughter's care and this shameful scandal.
"Doctors must be made accountable when they fail patients and parents.
"This is now an immense issue of public interest, with public confidence in our health service and the medical profession severely damaged."
Mr Justice O'Hara, who chaired the public inquiry into hyponatraemia-related deaths, concluded that the first inquest into Claire's death was wrong.
Her death was not reported to the coroner for eight years and her father said that between 2004 and 2006 misleading information had been given to the coroner.
Mr Justice O'Hara also said there was a cover-up into her death which was not referred to the coroner immediately to, in his words, "avoid scrutiny".
This prompted Attorney General John Larkin to open a fresh inquest into Claire's death.
For 21 years Claire's parents, Alan and Jennifer Roberts, have protested that the truth about their daughter's death was concealed and that the findings of the inquest into her death were wrong.
Last week, the BBC reported that a doctor involved in the Hyponatraemia Inquiry has, for the first time, been called before a fitness to practise tribunal.
An investigation takes place before a doctor can be called before a tribunal.
He was involved in the care of Lucy Crawford who died aged 17 months.
The BBC understands that at this stage four doctors are being investigated by the General Medical Council.