Nuon Chea, 88, known as "Brother Number Two", and former head of state Khieu Samphan, 83, were given life sentences in August for crimes against humanity.
The pair were the first top Khmer Rouge figures to be jailed from a regime responsible for the deaths of up to two million Cambodians from 1975-1979.
The resumption of proceedings brings to an end attempts by both men to boycott their second trial, in which they face genocide charges for the killings of ethnic Cham Muslims and Vietnamese as well as fresh counts of crimes against humanity and war crimes.
All lawyers for the defendants attended today's hearing.
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For health reasons Nuon Chea followed the proceedings from a holding cell, while Khieu Samphan sat in court alongside his defence team. Both men deny all charges.
Khieu Samphan's lawyers accused the court of trying to pressure his defence team against boycotting the trial by placing court-appointed counsel on standby.
"I have a right to a fair trial from a court which is impartial with my defence counsel of my own choice," Khieu Samphan told the judges.
The complex case against Nuon Chea and Khieu Samphan was split into a series of smaller trials in 2011 in a bid to obtain a fast verdict against the two men, both of whom are elderly.
The August convictions followed a two-year trial focused on the forced evacuation of around two million Cambodians from Phnom Penh into rural labour camps, and on murders at one execution site.
Both men have lodged appeals against their convictions.
Somewhere between 100-500,000 ethnic Cham Muslims and 20,000 Vietnamese were believed to have been killed during the regime's rule.