Abridged Series tend to have the main front runner creator voicing multiple actors, if he/she doesn't have a surplus of guest voices.
The cast of Half Full regularly talk to themselves. With cast member Matti Ryan playing Billy, Billy's father and mother together in multiple scenes and show creator Tom Kerin voicing the majority of background characters.
Tobuscus frequently voices multiple roles in his animated videos. In "Tobuscus Animated Christmas Adventures", he plays both himself and Santa Claus. In "Safety Torch", he plays himself and Little Timmy. Hilariously, in his Let's Play of The Legend of Zelda: Skyward Sword, he acts out the voices for all of the characters in the game.
Lego Pirate Misadventures has, for its first two episodes, all but a single character voiced by Mayhem. This trope is mentioned by name in the third episode, which coincidentally was the first episode to have an expanded roster for the other actor. Actually becomes noticeable when two characters voiced by Mayhem are conversing with each other, as they have fewer distinct pauses between their lines and may end up sounding a bit more like each other than normal.
In the Nostalgia Critic's review of Mortal Kombat: Annihilation, Doug has his beard shaved, and all three of his characters pop up in the intro with their own explanation of why they had their beard shaved (Nostalgia Critic shaved it because he made a bet that John McCain would win, That Guy shaved his because he thought the bet was about him posting his next video with the viewer responses within a week, and Chester just did it because he didn't want to be left out of the group). They quabble for a bit until NC dismisses Chester and then knocks out That Guy with a brick.
Davi in any conversation with Chuck, as well as the Doller Tree Clerk in episode six.
Spike while speaking with the CDU in human form based of Spike's body.
The Annoying Orange: Series creator Daneboe voices Orange, Pear, Midget Apple and Marshmallow, which frequently results in Talking To Himself when the four of them are together. Lampshaded in this video, which has Pear and Dane play Super Smash Bros. for Wii U together and Midget Apple comes in and states that nobody would be able to tell the two apart since they sound so similar (Pear's voice is the closest to Dane's voice).
The Trailer Trashed videos are essentially just five or so minutes of Dane talking to himself over a trailer, since the only ones that are trashing the trailer are Orange, Pear, Midget Apple and Marshmallow (except when Grapefruit trashes the trailer with them, taking Marshmallow's place).
Played with in an Achievement HunterLet's Play - the team for the Let's Play of RWBY: Grimm Eclipse is comprised of Michael and Lindsay Jones, Jeremy Dooley and Ryan Haywood and whenever Professor Port (who Ryan voices here and in the actual series) speaks up, Michael, Jeremy and Lindsay tell him to shut up. There's another point where Ruby whines and they tell Lindsay (who is her voice actress) to stop whining.
Given that Matthew Mercer plays every character that is not the main party (and even then, he'll fill in as needed when players can't make it), whenever NP Cs interact in Critical Role, this is the result.