The good twin/bad twin is a trope that goes back to works like The Man In The Iron Mask and Charlie Chaplin’s The Great Dictator. It’s also been utilized in myriad TV shows before like Gilligan’s Island, I Dream Of Jeannie, Bewitched, and Bonanza. For Friends’ Ursula and Phoebe, the twin plot was a bit more practical: Actor Lisa Kudrow had already appeared on NBC sitcom Mad About You as Ursula the flighty waitress, so the Friends writers made Phoebe Ursula’s much kinder twin, enabling crossovers between the two series.
As The Take points out in a near-half-hour-long video, Ursula and Phoebe are a typical example of using twins to “represent inverse moral views.” The twins both had a horrible upbringing—after their mother died by suicide, they were out on their own pretty early, with Phoebe’s tales of her early juvenile delinquency typically pretty horrifying. But Phoebe has decided to transcend her humble past to become a devoted loving friend, while Ursula is much more guarded emotionally, and frequently advantageous to get ahead (using Phoebe’s name for her career as a porn star, for example). So when Joey falls for Ursula (his reasoning: “Phoebe’s Phoebe, Ursula’s hot!”), it’s up to Phoebe to attempt to fix things when Joey’s heart gets broken. Ursula serves as the dark shadow under the brightness of Pheobe’s light-hearted benevolence.
So the Friends good/bad twins aren’t above posing as each other—an evil twin specialty, from horror movies to film noir—as well as showing two moral sides of the same situation. But The Take is a bit sympathetic toward Friends’ “evil twin”; after all, Ursula has had just as rough a time of it as Phoebe has, so she is easily the person Phoebe could have been. Maybe she’s just doing the best she can? Okay, but she still shouldn’t have sold Phoebe’s birth certificate to that Swedish runaway.