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As one of the judges on the show, Dewan, 41, tells PEOPLE exclusively that she will join her fiancé as he sings "Falling Slowly" from the musical Once, which the actor starred in and won a Tony Award for Best Actor in 2012.
"Steve hasn't performed 'Falling Slowly' since singing it on Broadway and we all thought it would be an incredibly special moment to have him sing while I danced," she says.
In a sneak peek of the performance, Kazee, 46, can be seen on screen, holding a guitar and singing the famous ballad. Dewan then starts her dance and is soon joined on stage by her partner, Robert Roldan.
"We love being able to share in a memory together for our children," Dewan says of her and Kazee's special moment. "So they will always have something to look back upon and have of us together."
"It's also fun to have these opportunities to play and create together that is a nice contrast to changing diapers and wrangling kids for bedtimes!" she continues.
Adds Dewan, "We were aware that getting the opportunity to share the stage together, to a song that really impacted both of our lives, was too great an opportunity to pass up!"
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The couple's connection to the song dates all the way back to when Kazee was starring in Once on Broadway in 2012.
Dewan revealed she first connected with her now-fiancé after meeting Kazee backstage with her mother following his performance in the show.
"I had met Steve years ago in a flash, it must've been five minutes," she told PEOPLE in October 2019. "I saw him perform in Once on Broadway and I was just totally blown away by his talent. My mom was with me, and we got to go backstage and meet the actors. He just had a cool [energy] about him."
"Obviously I was married and so we never spoke or saw each other again for six years, but I never forgot the moment," Dewan explained. "And then six years later, obviously it becomes public knowledge that we're separated and he contacted me."
"I had kind of come to terms with, 'Okay, I need to accept that I need to start a new life.' I had no idea what I was looking for, and I was not at all looking for any kind of relationship," she continued. "It was nothing I was looking for, but it was everything I needed, if that makes sense."
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The World of Dance alum said they started with a phone call, which lasted for hours. Dewan also noted how there was an "immediate connection," and even more so when they met in person.
"It was an instant connection when we met each other. All of a sudden," she said with a smile. "There was no way we could avoid it. So in my eyes it was truly, completely meant to be. There was like a lightning bolt behind it."
After the duo officially started dating in October 2018, they revealed one year later they were expecting their first child together in an interview with PEOPLE. Kazee popped the question in February 2020, and their son, Callum, now-2, was born the following month.
"I wasn't looking for it," Dewan told PEOPLE of her romance with Kazee. "But it sort of took me by complete storm. It happened when I least expected it."
Come Dance with Me airs Fridays at 8 p.m. ET on CBS.