Colbert and Fallon responded by broadcasting a mock video chat, in which they addressed each other as "lowlife" and "lost soul".
"Hey lost soul", Colbert replies, beaming.
"I$3 heard he said some pretty bad stuff about us", Fallon told Colbert about Trump's SC rally.
The two hosts then proceeded to call Conan O'Brien, who appeared to have no idea that Trump was president. He says, "If we're not careful, this could start to get ugly". "If I couldn't I'd be in big trouble ... they're not talented people".
He also said that he recently had to tell Fallon to "be a man" and not apologize for messing up Trump's hair during a 2016 appearance.
The joint cold open aired at the beginning of both CBS' The Late Show and NBC's The Tonight Show, making for a rare effort across networks. "Then he went after someone else on TV you know and love", Colbert said before playing a video of Trump hurling insults at "the guy on CBS".
"Then I thought, 'Wait, shouldn't he have more important things to do?'" Fallon said to laughs.
Fallon said he never called Trump to receive "monster ratings".
"Hey lost soul. What are you up to?"
On Monday, Fallon seemed perplexed that Trump had attacked him on social media. "Conan", O'Brien's late-night TBS show, didn't feature an original episode Tuesday. "He's the president! What are you doing?" It - sigh - is like the Avengers: Infinity War of late night Trump comedy.
"I remember when [the] US president was too busy to pick fights". One commenter wrote, "I don't blame Fallon on the way he feels".