Taken Up to 11 with Pierre Coffin, who does the voices for every one of the Minions. (And the Spin-Off film has 899 of them!)
The third movie introduces Gru's brother, Dru, who is also voiced by Steve Carrel, resulting in quite a bit of use of this Trope in the movie. (Carrel has actually discussed the challenges of having to play such roles, seen here.)
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.
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 Alvin, Simon, and Theodore. In the post 1980s franchise, Theodore was voiced by Janice Karman (voice of the aforementioned Chipettes).
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.
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 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.
Ferngully The Last Rainforest has another impressive example of this in it's Spanish dub: 32 speaking parts, even the girls, were done by comedian Angel Garo.
In Cinderella the King and the Grand Duke are both voiced by Luis Van Rooten.
In "Tom and Jerry: Back To Oz" The Wizard has brief conversation with Droopy and Butch who were serving as the Nome King's henchmen, all three of them voiced by Joe Alaskey in one of his final performances.
Frozen: An In-Universe case, wherein Kristoff will turn to Sven for advice. But as Sven is a reindeer and can't speak, Kristoff has to provide a "voice" for him. Hilariously, it's always Sven who "wins" any argument.