In the Brazilian dub of Beauty and the Beast, where Beast and Gaston are voiced by the same actor. But the fight between the characters is brief in dialogue (the only line the Beast says to Gaston is "Go away").
Eight Crazy Nights has Adam Sandler voicing Davey, Whitey, Elanor, and the helpful forest deer; the first three talk to each other a lot because Whitey and Elanor take in Davey because he has no place to live after his trailer was burned down.
In Lady and the Tramp Bill Thompson plays five characters Jock the Scottish terrier, a bulldog, Joe the Italian cook, a dachshund, and a police officer; the bulldog and the dachshund talk to each other in a few scenes.
You know the client who wanted radio tubes from Elmo St. Peters? Same guy voices Lampy.
In Alicein Wonderland, J. Pat O'Malley voices both Tweedledee and Tweedledum, as well as both the Walrus and the Carpenter. Bill Thompson also voices both the Dodo and the White Rabbit, who share an extended scene together.
One of the older movies features the Chipettes; you can find videos on Youtube showing that if you slow down the audio, it's just one woman voicing all three.
Likewise Alvin, Simon, and David are all voiced by Ross Bagdasarian Jr., and in the original records and cartoon his father Ross Bagdasarian Sr. voiced David as well as all Alvin and Simon. Theodore was voiced by Janice Karman (voice of the Chipettes).
An early example would be fact that in Pinocchio, Charles Judels actually voiced both Stromboli and the Coachman, both villains that became Karma Houdinis at the end of the film.
Lea Salonga actually does the singing voice of both Jasmine and Mulan. This is especially noticeable in several Disney Princess CDs and music videos where the two are both singing at the same time.
If you listen very closely at the very beginning of Atlantis: The Lost Empire, you can actually hear Cree Summer voicing the former Queen of Atlantis. Cree is normally the voice of Kida, who ends up becoming a queen at the end of the film, and is the only princess created by Disney to ever actually become one.
In the sequel, both Luigi's Aunt Topolino and the Queen of England are voiced by the same actress, as with the actors playing Siddeley and Leland Turbo and Fillmore and Tony Trihull.
Subverted in The Little Mermaid II where Pat Carroll (the voice of Ursula) actually does the voice of Morgana, Ursula's younger and skinnier sister and the film's main antagonist, but Ursula is now long dead when the sequel took place (which explains why Morgana was the villain in the first place).
BIONICLE 2: Legends of Metru Nui has Lhikan, Krekka (Both voiced by Michael Dobson), Onewa and Matau (both voiced by Brian Drummond). Expect a fair amount of this trope happening amongst them. Web of Shadows continues the trend with Onewa and Matau again.
In Spirited Away, the sisters Yubaba and Zeniba are voiced by the same actress both in Japanese and English. The English actress, Suzanne Pleshette, indicated that she changed the differences between the two voices; the Japanese voice of Zeniba was lower-pitched, whereas she changed her voice to be higher-pitched.
J. Pat O'Malley actually voiced both Colonel the sheepdog and Horace Badun in 101 Dalmatians and both Colonel Hathi the elephant and Buzzy the vulture in The Jungle Book.
In "The Whale Who Wanted To Sing At the Met," the tragic final short of "Make Mine Music," Nelson Eddy speaks and sings all the voices. EVERY. LAST. ONE. OF. THEM.
Hoodwinked!: Flippers' interview with Kirk would have been David Ogden Stiers interviewing himself had the producers not recast Jim Belushi in Kirk's role.
In Who Framed Roger Rabbit, Charles Fleischer voiced both Roger and Benny the Cab, who get into two different arguments.
Driving through downtown L.A
Benny: Hey Roger, what do you call the middle of a song?
Roger: Gee, I don't know...A BRIDGE!!!
The "Johnny Appleseed" segment from Melody Time has Dennis Day voice both Johnny and his Guardian Angel.