In the powerful chat, the Duchess listened intently as both Manfred and Zigi shared their incredible stories.
The two brave men met back in 1944 at a concentration camp and did not see each other again until years later.
Manfred opened up about his harrowing escape from the camps.
"We were shuffling, single-file, forward until each of us faced an SS man who would say left or right. And, by that time, we knew that left meant death today and right meant survive, until the next selection at least".
As they were lining up, Manfred explained how a boy behind him whispered to tell the Nazis he was 17. And, although he was 14 at the time, Manfred took his advice and he was "sent to be spared".
The 90-year-old went on to say he's never seen that man again and he doesn't know which way he was sent but "he's in (his) thoughts as (his) angel".
Kate listened intently and asked why the age 17 was valued. Manfred revealed it was because they could "exploit the kids for slave labour".
Zigi also bravely opened up about his experience.
In 1944, he moved to Germany with the metal factory he was working with to "escape the Russians".
"When I got to the station I said 'mother, I can't see any trains', she said 'they're standing right in front of you'. I said 'that's not for us, that's for animals.'"
The 91-year-old went on to reveal how people were put on the train, with no place to sit down, and taken to different camps. Ninety per cent of the group were killed.
While Zigi explained he wasn't given a number on his arm, he was still classed as number 84,303.
"I can't forget it, I want to get rid of it," he said.
Eventually Zigi was taken to the camp where Manfred was and then to more camps after that until he received a letter from his mother and was able to escape to England.
An enraptured Kate was extremely moved by both men's stories, and thanked them both for their bravery and generosity in sharing such harrowing memories.
“The stories you have both shared with me today and your dedication to educating the next generation, the younger generation, about your experiences and horror of the Holocaust shows extreme strength and such bravery," Kate said.
"And it’s so important and so inspirational so thank you once again for sharing your stories with me and for all the work you do."
The Kensington royal Instagram page also made a commemorative post.
The caption read, "Together on 27th January, the anniversary of the liberation of Auschwitz-Birkenau, the largest Nazi death camp, we bear witness for those who endured genocide, and honour the survivors and all those whose lives were changed beyond recognition.
"We must never forget."