Speaking about the pair's relationship, a source exclusively tells PEOPLE, "Amy was going through a heartache and TJ was very much there for her and helpful, and that is when they started something."
The GMA 3 co-anchors' relationship was publicly revealed on Wednesday when photos surfaced of them holding hands in a car, enjoying a vacation in upstate New York and cozying up at a bar in New York City.
The insider says that Robach, 49, and Holmes, 45, "had no shame going out together and have been publicly doing this for much longer than a month."
Noam Galai/Getty T.J. Holmes and Amy Robach
The source adds that the pair's relationship was "the worst kept secret in ABC News."
"Staffers are very surprised it took this long to come out," the insider adds. "Everyone was turning a blind eye because they didn't want the controversy surrounding the show."
A rep at ABC News did not immediately respond to PEOPLE's request for comment.
Bryan Bedder/New York Road Runners via Getty
"This was two consenting adults who were each separated. They both broke up with their spouses in August within weeks of each other," the insider said of Robach and Holmes, who has been married to attorney Marilee Fiebig since 2010. "The relationship didn't start until after that."
The insider said that Robach, who married actor Andrew Shue in 2010, has "got nothing to hide," adding, "They were both separated so they felt very comfortable dating in the open after that. Their spouses had moved out, even! So they were not hiding anything."
Holmes joined the GMA team in 2014 and immediately struck up a close connection with Robach. She previously told PEOPLE about their friendship.
"[They] said, 'Hey, we want to bring on a co-anchor. What do you think of T.J. Holmes?' And I almost fell out of my chair," she said. "My God. You couldn't have picked a better person to ask me what I thought of, because we've been trying to figure out a way to work together for literally the last five years."
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Robach also told PEOPLE about their double dates with Shue and Fiebig. "The moment he started at ABC, I think we just clicked. We've gone on tons of double dates with our spouses and my daughters babysit his daughters."