The pair call a clueless O'Brien, who is surprised to learn Trump is the president. In the piece, Fallon said he regretted having candidate Trump as a guest on his show and choosing to muss his hair.
On Colbert's show, the short video opened with Colbert writing a joke and congratulating himself - "Great monologue!" - before answering a video call from Fallon. That's when Colbert and Fallon made a decision to join forces and take the president down a peg or two. Trump said Fallon was always waiting for him when he appeared on his show prior to winning the White House and treated him with respect.
"Hey lost soul", Colbert replies, beaming.
"Really? That doesn't sound like him", said Colbert.
O'Brien then joins the conversation, unaware that Trump was even elected president.
"Mostly whimpering", Fallon said, eyes downcast as he referenced the president's recent tweet lambasting him for his apology. "As you may have heard, last night, the president of the United States went after me on Twitter". Colbert responds by calling him "lost soul". "I heard he said we're all no-talent, low-life lost souls". "That's Conan", Colbert said, before placing a call to O'Brien. "The real estate guy who sells steaks?" he asks in disbelief. "He's president? How's he doing?" "If we're not civil, this thing could start to get ugly".
After Fallon and Colbert hang up on Conan, the two discuss lunch plans."Yeah, where do you want to eat?"
The US chain has also been attacked by Trump after his press secretary Sarah Sanders was refused service at one of its branches.
"Red Hen!" Colbert said, closing the segment and leading into the opening credits.