The clip has been viewed nearly 3 million times since it was posted on YouTube Friday.
At one point in the video, Fallon's Sara character challenges Grande's Alexa to an "Ew!" sing-off.
Sara garbles a bunch of notes before passing off to Alexa.
"I can't top that," Alexa deadpanned.
"You have to try," Sara encouraged her.
After Alexa sings a pitch-perfect "Ew!", Sara tells her, "That was OK, I guess," and shrugs.
The pair then broke into giggles as they debated whether actor Richard Dreyfuss was cool or Ew!, then toasted their friendship with Shamrock Shakes from McDonald's.