Bou Meng and Chhum Mey spend less time at the trial of Khmer Rouge leaders these days, and more time at the torture center they both survived.
The UN sent its expert for the court to Cambodia this week to meet with government and court officials over a UN-appointed judge.
Overcoming such trauma is not easy and going back into the past does not help either, Sam Keo said.
Bou Meng and Chhum Mey spend less time at the trial of Khmer Rouge leaders these days, and more time at S-21 museum.
UN Secretary-General Ban Ki-moon nominated Kasper-Ansermet to replace a German judge who resigned last year.
The Documentation Center has collected nearly 1 million documents on the Khmer Rouge since 1995.
Kasper-Ansermet was to have filled a vacancy created by the departure of German judge Siegfried Blunt - who resigned late last year.
A UN spokesman told VOA Khmer this week the government has an obligation to fill vacant posts at the court.
The UN appointed a reserve judge to the position, Laurent Kasper-Ansermet, but that appointment has not been confirmed.
This comes a day after defense attorneys say they want to summon the head of a leading genocide research organization before the court.
The Documentation Center has provided around 500,000 documents to the tribunal since its inception.
Jailed Khmer Rouge leader Khieu Samphan refused to answer questions from the UN-backed tribunal on Thursday.