She plays both of them. When they are on the same screen they shoot the scene twice, when she is being Phoebe, a back up actor will be Ursala so Phoebe has someone to talk to, and then vice versa. They then combine the two shots into one
The character Ursula is from the tv mostra Mad about you, which Lisa was on at the time, then since Friends and Mad about te were in the same city they decided to put Ursula on Friends as well, so they Lisa played to different characters, a lot of celebritàs have done that kind of acting, Lindsay Lohan in The Parent Trap and I know who killed me, Lucy Hale in an episode of CSI - Scena del crimine Miami, Nina Dobrev in The Vampire Diaries, Hilary Duff in The Lizzie MaGuire movie. It's a common thing that celebritàs have done.
