Nuon Chea, 86, and Khieu Samphan, 81, are accused of war crimes, crimes against humanity and genocide, the Indo-Asian News Services reported.
Nuon was known as Brother Number 2 to Brother 1, Pol Pot, during the short but deadly Khmer Rouge regime from 1975-79.
Pol Pot -- the regime's most wanted man -- was captured in 1997, but died while under house arrest in 1998.
Although not proved, it's been suggested that Pol Pot either was poisoned or committed suicide.
Hundreds of thousands of Cambodians also fled the country into exile and uncertain refugee status in the period that became known as the Killing Fields.
Nuon was the deputy secretary of the Communist Party during the time and Khieu was president of Democratic Kampuchea -- the name given by the Khmer Rouge to Cambodia during their regime.
"The closing statements will last until Oct. 31 and a verdict is expected in the first half of 2014," Dim Sovannarom, chief of the public affairs section at the United Nations-backed Extraordinary Chambers in the Courts of Cambodia, said.
The tribunal was set up in 2006. More than 90 people have given evidence in the lengthy and controversial Nuon and Khieu trial that started in November 2011, IANS reported.
Ieng Sary, the regime's former foreign minister, also was on trial. But in March Ieng died at the age of 87 due to illness.
The case against his wife Ieng Thirith, 81, ex-minister of social affairs during the regime, was suspended in September 2012 after the court ruled dementia had left her unfit to stand trial.
Nuon is accused specifically of leading the Year-Zero plan, a mass forced migration of people from cities to work as peasants on communal farms in the countryside.
The policy included the abolition of money and private property, as well as banning religion.
An invading Vietnamese army eventually overthrew the regime whose leaders and cadres, including Pol Pot, fled into the jungle where they remained for years.
All the defendants have pleaded not guilty.
The tribunal has sentenced only one defendant -- Kaing Guek Eav, former head of the feared Tuol Sleng prison.
Kaing, also known as Duch, was given life in prison for overseeing the deaths of around 15,000 people under the regime.
During questioning in March last year, Duch described that his immediate superiors instructed him in interrogation methods.
The Voice of America reported at the time that Duch said he was shown how to apply electrical wire to bodies, methods of water-boarding and suffocation
The trial has been plagued by disruptions and disagreements between international judges and Cambodian judges, the BBC reported.
In September court workers went on strike over unpaid wages.
Rights groups and campaigners have criticized the pace of the trial and voiced fears that the ages of its Nuon and Thieu could mean they are treated leniently, the BBC reported.