In Dota2, it's possible to have a match with everyone voiced by Nolan North.
In Half-Life 2, there is only one voice actor each for all male and female rebels. This means any time two same-sex rebels have a conversation, it's with the same voice.
The Italian version of Rayman 3: Hoodlum Havoc featured Dario Oppido who voices: Rayman, Globox as well as Reflux, resulting in quite some moments of him actually talking to himself.
The English version of Beyond Good & Evil had far fewer voice actors than any other version, resulting in some interesting conversations. Most of the side female characters are quite clearly the same woman, especially obviously in the case of The Faceless, the Science Center woman, and the governor. Even funnier, the Elite Mooks, the Alpha Sections, obviously have the same VA as Playful Hacker Nino, leading one to wonder just whose side that guy is on.
Conkers Bad Fur Day has dozens of characters with major speaking roles, yet only three people do the voices for all of them, and one of those three does only one voice. Chris Seavor actually voiced over forty characters; every single male part except the Great Mighty Poo. Now that's some incredible range.
Disgaea: Hour of Darkness has one man playing General Carter, Thursday, and Archangel Vulcanus. While Thursday never has any lines with either of the other characters, both Vulcanus and Carter get a whole scene together.
In The Elder Scrolls IV: Oblivion, one actor performs every character of the same race and gender, sometimes doubling up, and they rarely try to change it up by character. If you hear a female wood elf and a female dunmer talking to one another, they sound like the exact same person, sometimes repeating bits of the same conversation back and forth. The exception is the characters voiced by Patrick Stewart, Terence Stamp and Sean Bean. This is mainly due to the rumour function, which is randomised. If the actors gave each character distinct vocal traits, they'd have to re-record the rumours hundreds of times more. This is exemplified with the beggar in Bruma who actually does sound weak and decrepit, but switches to upbeat and healthy as soon as you ask about the weather.
In the Zero Punctuation review, Yahtzee's biggest gripe with the game was this, claiming that it completely yanked the immersion of the game out, as you're having a fabulous adventure of some sort, only to hear one of the NPCs' stale voice acting, sounding exactly like a guy you just killed or bought stuff from, and you're suddenly back in your room, staring at a screen and listening to shitty voice acting.
The Elder Scrolls V: Skyrim had more voice actors in the game than Oblivion, but still ran into this trope from time to time. Since there were fewer completely randomized NPC conversations, they sound more natural (especially since Bethesda had the voice actors use different inflections for each major character they voiced), but it's especially prominent with some of the non-Nord races. Having two dark elf males converse with each other can quickly turn into a Keith Szarabajka party (this is most noticeable on the Vaermina Daedric Quest in Dawnstar).
Many of the voice actors in the Fatal Frame series play multiple roles as attacking spirits. In Fatal Frame 2 and Fatal Frame 3, this is done deliberately, doubling several seiyuu as both major protagonists and major antagonists, to blur identities as the living characters are drawn deeper into the ghosts' stories.
Most NPC's in the Spanish version of Fable I are voiced by the same actor. When there's more than a few characters around you it can get pretty ridiculous.
Fallout 3 and Fallout: New Vegas have the same problem, running on the same engine (and with Oblivion and Fallout 3 being made by the same company). Liam Neeson and Matthew Perry both voice-act one character each, and each of the companions have their own unique voice (except Charon, who has the generic "male ghoul" voice of Mike Rosson, and Jericho, who has the same voice as the male Raiders; i.e. James Lewis). Pretty much everyone else boils down to the same handful of voices. It's most jarring with Ghouls, who, with the exceptions of Desmond Lockheart (Jeff Baker), Plik (uncredited), Jason Bright (Graham Cuthbertson), Raul Tejada (Danny Trejo) and Dean Domino (Barry Dennen), have 1 male voice and 1 female voice per game. One of the more egregrious offenders is Gus Travers, who voices Flak, the Rivet City security guards, and Armitage, all of which can appear in the same room at the same time.
Fallout: New Vegas seems to introduce the player to a world inhabited by clones of Gregory Alan Williams(e.g. Bruce Isaac and the Lonesome Drifter, both involved in the Talent Pool quest) and Katherine Pawlak(e.g. Sunny Smiles and an unnamed Goodsprings settler who inhabits the same house). Conversely, most of the characters from the DLC's have unique voice actors, and Super Mutants have more varied voices than in FO 3, where they all had the voice of Wes Johnson, including Fawkes.
In Old World Blues, Veronica Belmont voices both Light Switches as well as the Stealth Suit Mk. II.
In Sam And Max Season One and Two, Roger L. Jackson voices Abe Lincoln, Grandpa Stinky, the Mariachis, Satan and "every deep voice" according to the commentaries.
William Kasten also voices Max, Jurgen and the Mariachi spaceship's computer AI, in four different styles (Politically Correct, Abusive, Suggestive and Passive-Aggressive).
Then there's Jared Emerson-Johnson, who voices three of the C.O.P.S.note The fourth, Chip, only speaks in electronic beeps and blips. They're never seen apart, so Talking to Himself occurs very frequently.
Joey Camen voices Bosco, Jimmy Two Teeth and his family.
And in the E3 2006 trailer, David Knowlin noticeably voiced both Sam and Max.
A particularly terrifying example of this trope takes place in the first level of Knights of the Old Republic II, as HK-50 relates the Maintenance Officer's agonized last words.
Charles Martinet does the voice of not only both of the Mario Bros., but their evil counterparts Wario and Waluigi, their baby versions, and Toadsworth. And the Boos all appear to be voiced by the same actor who plays Bowser. Do you know how you actually figure this out? If you play a Boo's laughter very slowly, it'll actually sound like Bowser, and vice versa!
Martinet also voices Donkey Kong in the original Mario Party.
Even more impressive, he provided the voice of every character in Super Punch-Out!!.
In Xenosaga, both Albedo and Gaignun are voiced by Crispin Freeman. Their voices do sound somewhat similar, but they are 'brothers', so this is acceptable. However, the dramatic, split-screen, mental discussion the two hold with one another in Episode II loses some of its drama when one remembers its just one man talking to himself. This was most likely a deliberate casting choice to reflect their origins, since in the Japanese version, Yamadera Kouichi is also double-cast for the same roles.
Similarly, KOS-MOS and T-elos in Episode III have the same voice actress in both Japanese and English.
Xenoblade follows suit with a particularly noteworthy example, giving Shulk and Zanza the same voice actor in both the original and the dub. Name one other game where the final boss battle is one long instance of this trope. (And the Final Boss isn't the hero's dark side or something similar.
Luke and Asch are voiced by the same voice actor and reasonably so, as one is a clone of the other. Thus, a climactic scene near the end of the game where their enmity plays out is possibly more impressive when you realize it's just one guy screaming at himself.
In the same vein, Fon Master Ion and Sync the Tempest are also voiced by the same person, as they are both clones of the same person. This is actually used by Sync to deal a very low blow towards Anise at one point.
About half of the characters from the 3DS remake of Star Fox 64 are voiced by one guy: Jaz Adams provides the voices of Peppy, General Pepper, Andross, Wolf, Leon, and Pigma. Meanwhile, Mark Lund provides the voices of Falco and Andrew.
F/Final the first fully voiced SRW game probably has the most of this. To save money perhaps nearly all the original and MasouKishin characters are voiced by V As that all had another role in game and usually a main character one as well. (Hikaru Midorikawa for example is Heero Yuy, and also was cast as Masaki Andoh) Although most of these characters haven't appeared since, a few of them have and nearly eclipse the other characters in popularity. Masaki for example became one of Midorikawa's favorite roles, despite initially only getting the part to save time while he was recording Heero.
The German version of faces the problem that most male heroes in Disney movies are voiced by the same person in German, resulting in Aladdin, Prince Eric, and Hercules sharing the same voice. Luckily, they never really met in the game.
And in the English version, there's Corey Burton in both games with five to seven roles. Thankfully, they still never meet.
In the Japanese version we have K˘ichi Yamadera, who voices Donald Duck, Genie, Sebastian, Beast, Mushu, and Stitch.
Don't forget the most literal example of this in RE:Chain of Memories, with David Gallagher playing both Riku and the Riku Replica.
In the original Warriors Orochi, when Zhuge Liang sniggers at Zhao Yun's inability to comprehend why The Strategist is working under Orochi, it's the same seiyu (Masaya Onosaka) acting out this trope. Also, since the game has a character switching system, it's possible to hear the same seiyuu doing the "swap-in" lines for two very different characters. By hearing them, you won't believe that Date Masamune and Fuma Kotaro are done by the same person.
Of course, this is also the case in their parent games, Dynasty Warriors and Samurai Warriors in both the Japanese and English voice tracks, owing to their own massive casts. For example, Lex Lang voiced both Zhuge Liang and Wei Yan at one point, as well as Sima Yi.
Before the 4Kids voice actors took over, Sonic's voice actor Ryan Drummond also voiced Metal Sonic in Sonic Heroes.
After they changed voices AGAIN, now in Sonic Free Riders, both Tails and Wave the Swallow are played by Kate Higgins, both Storm the Albatross and Knuckles are played by Travis Willingham, and both Omochao and Blaze are voiced by Laura Bailey.
BlazBlue has two examples of this trope in both English and Japanese, and one in English only. Since knowledge of who voiced who in these cases would spoil the story, the characters' Voice Actors/Actresses are not listed in the credits. The characters, in question, are as follows: Jin Kisaragi and Hakumen are both voiced by David Vincent and Tetsuya Kakihara; Noel Vermillion, Nu-13, Lambda-11, and Mu-12 are all voiced by Cristina Vee and Kanako Kondo; and Tsubaki Yayoi and Kokonoe are both voiced by Julie Ann Taylor.
The Thief series, at least at first, only had a handful of voice actors, which resulted in Stephen Russell playing Garrett (the Deadpan SnarkerAnti-Hero), Benny (the resident Butt Monkey and Running Gag), Father Karras (a Big Bad) and many other extras and bit parts, all of which are very different characters with distinct voices (fortunately). At one point, he complains about himself prattling on.
Chris Metzen, best known as a story-writer, also voice acts two characters in Warcraft III. These characters, Thrall and Rexxar, happen to get a lot of dialogue in the Expansion Pack's Orc campaign.
Psychonauts. Andre Sogliuzzo voices both halves of a split personality, Fred/Napoleon Bonaparte, who argues with himself. Napoleon has an outrageous French accent. Fred sounds like a classic slacker.
In Anarchy Reigns, Stephanie Sheh voices all three of the Rin sisters. Two of them, Ai Rin and Fei Rin, talk amongst themselves in the story mode.
In Star Wars: The Force Unleashed, Sam Witwer (somehow) voices both Villain Protagonist Galen Marek and Emperor Palpatine. In the final battle, he ends up having rather lengthy conversations with himself, switching back and forth between what is apparently his natural voice (Galen) and an excellent impression of Ian McDiarmid's Large Ham-ish Palpatine voice.
In a bit of Team Fortress 2 irony, the Pyro and the Spy share a voice actor, as do the Heavy and the Demoman. Both pairs have little to no dialogue that directly refers to the other, save for the Pyro calling "Spy!" and the Spy's Pyro-domination quotes since the Sniper vs. Spy update, but many fledgling Spies tend to be revealed as moles by enemy Pyros, and Heavies tend to be perfect prey for a Demoman's sticky bomb trap.
In a more straight example, Nolan North voices both Redmond and Blutarch Mann in the 2013 Scream Fortress event, and they constantly taunt and bicker with each other. Also add Zepheniah giving either brother a compliment, who's also voiced by Nolan North.
Nolan North also voices Merasmus and the Bombinomicon, Halloween characters from 2012.
Gary Schwartz (Heavy and Demoman) sort of voices MONOCULUS!, since its voice is just Demoman voice clips greatly lowered in pitch.
Robin Atkin Downes voices the Medic, the Second Opinion "evil Medic" voice, and the shoulder-sitting Archimedes the Undying.
There is a scene in Gabriel Knight: Sins of the Fathers where Southerner Gabriel Knight (portrayed by Tim Curry), posing as Mosely, first visits Malia Gedde's mansion and has to negotiate his way past her English butler Robert, also voiced by Curry. Especially funny after Malia has Gabriel thrown out, leading to this little piece of dialogue:
Gabriel:(very sarcastic) Thank you very much! I had a looovely time! Aw shit...
At the same time, it averts this with Mamiko Noto, offering no interaction between Haruka and Bonus BossHecate (despite Haruka being a non-playable party member, so technically they do meet). Ditto for Kazuhiko Inoue and Kimiko Koyama, who both reprise roles across two different series; their respective characters don't meet at all
Oracle Of Tao: the voice actress for Ambrosia, the lead character, also plays her evil twin (which is an example, even though they are technically the same role, since the evil twin is a Literal Split Personality). Oddly, there's an inversion, as she is supposed to be one and the same with God, but this role has its own actor.
In fact, Street Fighter IV is probably the first game where everyone is voiced by different actors. The Alpha series had Toshiyuki Morikawa as Ryu and Charlie, Tetsuya Iwanaga as Ken and Guy, Koichi Yamadera as Balrog and Cody, Akiko Komoto as Cammy, Juni, and Juli, Wataru Takagi as Zangief, Sodom, Birdie, Gen, and Adon, and others. Street Fighter III has Len Carlson as Q and Hugo, and Lawrence Bayne as Gill, Urien, Necro, and Twelve. The original Street Fighter II used the same sounds for Ryu and Ken, and everyone (save Chun-Li, for obvious reasons) had the same death scream.
The Japanese version of IV averts this trope completely. The English version mostly averts it as well, with the exception of Taliesin Jaffe, who voices Blanka and Adon. It gets better when one realizes that he's the voice director, so he was also directing himself.
Initially, Patrick Seitz was the only voice actor in Street Fighter X Tekken to voice two characters, who are Hugo and Bob. After the DLC characters were released, four more voice actors joined Seitz. They are Tomokazu Seki (Yoshimitsu and JPN Bryan), Junichi Suwabe (JPN Vega AKA Claw and JPN Lars), Kenichirou Matsuda (JPN Raven and JPN JACK-X), and Jordan Byrne (ENG Kazuya and ENG JACK-X).
Even if the voice-acting is minimalistic to barely existent, in The Legend of Zelda: The Wind Waker, Link's voice (consisting of battle-cries, screams of agony and surprise, a "Hey!" and a rarely used "come on!") is done by the same woman who does his sister Aryll's voice (consisting of screams of fear, some giggles and a loud "Ooooooy!!!")
The consciences in Black & White are voiced by the same actor, and frequently disagree between eachother on what the player should do in any situation. This is lampshaded in the downloadable outtakes, where the voice actor gradually morphs his voice from the good conscience's to the bad one's and back, in the middle of a conversation between the two.
The Metal Gear Solid series manages to avert this for most part, even though many of the voice actors in both, the Japanese and English versions, voiced various different characters thorough the games. A notable exception in which this trope is played straight is in the Japanese version of Metal Gear Solid 2, in which Akio Ohtsuka played both, Solid and Solidus Snake.
Notably averted in Guns of the Patriots, where the older Big Boss has a new voice actor to avoid having the big payoff of the entire series wind up as a conversation between David Hayter and David Hayter.
The voice of The Medic who performs surgery on Paz is played by Snake's Other Darrin, Kiefer Sutherland, with some subtle digital pitch shifting to disguise it. Some fans came up with complicated Epileptic Trees based on this, but it's unlikely to have any significance.
Revolver Ocelot, Liquid Snake, Colonel Campbell and Psycho Mantis's cameos in the quiz show segment of "Deja Vu" (and Psycho Mantis's cameo in "Deja Vu" itself) are all provided by Kaz's voice actor Robin Atkin Downes doing impressions of the original cast. All of them interact both with each other and with Kaz himself.
Michael Bell is Raziel, and occasional minor roles, like the first of Raziel's brothers he kills in Soul Reaver. Bell being a Man of a Thousand Voices, it's hard to tell without knowing beforehand. This happens literally in Soul Reaver 2 when two versions of Raziel from different time periods meet.
The series purposefully averted this trope on one occasion; Tony Jay voiced Mortanius in the original game, and then the Elder God for the next two, spurring no small amount of speculation on how the characters could be related. When the two characters finally ended up in the same game, a different actor was cast as Mortanius as a way of telling the audience that the characters aren't meant to have a connection, because the plot is so crazy at times that a simple absence of a confirmation that they're related would be seen as a confirmation.
In the series, the English-language releases of the games have Christopher Robin Miller tripling up as not only the titular character, but also Inspector Chelmey and his "arch-nemesis" Don Paolo. He's apparently skillful enough that nobody realized it.
The games also have Lani Minella as not only the titular character's apprentice, but also their companion Flora, "guest characters" Sophia and Katia in the second game, Claire and Celeste in the third game, plus Emmy Altava and "guest" Arianna in the fourth game (and some more female smaller parts). Minella and Miller voice more characters in the series than all other voice actors combined.
Main character, Pharos, Ryoji Mochizuki, and Nyx Avatar share the same VA in both English and Japanese. All 4 are more or less the same person.
This also applies to Shinjiro and Jin (Grant George) and Chidori and Ken (Mona Marshall) while in Persona 4, both Nanako and the gas station attendant are voiced by Karen Strassman and both the main character and Adachi are voiced by Johnny Yong Bosch. However, at no point do any of these characters directly talk to each other; as Shinjiro and Jin only have one scene together where Jin doesn't talk, Chidori and Ken don't have any scenes together, and the silent protagonist obviously doesn't talk. Nanako and the gas station have one scene together where they both talk, but not to each other.
Then there's the character's shadows, which naturally share a voice with the normal character. Most of them sound like themselves, though with an extra shot of jerkass as they bully their normal selves with everything they try to hide, but Shadow Teddie shows Dave Wittenberghas some incredible range.
Like above, you can also potentially have Karen Strassman talking with herself as Aigis and Nanako, but only if you get Nanako's navigator DLC.
American video gamers are so used to voice actors being tasked with several roles in a single game that it's no surprise to hear the same voice actor for both Warden Quincy Sharp and the Spirit of Amadeus Arkham in Batman: Arkham Asylum; the same actor also has a few other minor roles. What makes this noteworthy is that the two characters of Nominal Importance are related. Bravo, Eidos, using our knowledge of the industry to lead us to ignore the most obvious solution!
The English version of Snatcher narrowly avoids this trope literally, even though there's only three male voice actors (Jeff Lupetin, Jim Parks, and Ray Van Steen) out of seven in the whole game for all the characters.
Michael Bell also voices multiple characters in the Ratchet & Clank series, with minimal interaction, but at one point narrates a mini game featuring another of his characters. Similarly, both Captain Qwark and his mascot Skrunch are voiced by Jim Ward.
In Halo, the pilot of the escape pod Master Chief rides in has the same voice as Cortana (Jen Taylor). Also, Pete Stacker voiced one of the Sergeants as well as Capt. Keyes. And sometimes, there's two or more Red Shirt Marines with the same voice onscreen at the same time.
In Backyard Skateboarding, Marky, Pete, Pablo, and Erik are voiced by the same woman. The latter three can try Marky's challenge, so when they talk, it is Talking To Herself. This is averted in the other games, however, as there is no dialogue.
Strong Bad's Cool Game for Attractive People naturally has most of the characters voiced by Matt Chapman (see below example in Web Animation), but also includes an in-universe example. In Dangeresque 3, Homestar plays two characters. In one scene, the two characters talk to each other, resulting in a really obviously bad split-screen shot (though in all the other scenes involving both characters, Strong Sad serves as a Stunt Double for one.)
Lorne Lanning is not only the co-founder of Odd World Inhabitants and director of all of their games, but he also voiced 80% of the cast in those games.
Singularity uses this to toy with the player; Nolan North lends his distinctive voice to two characters, but one of them only has one line, and he delivers it when he can't be clearly seen; if you notice his voice and you don't assume it's the other character, you'll assume it's just this trope.
Nolan North voices more than a few pedestrians in Mafia II which occasionally leads to him talking to himself as demonstrated in this video.
The Phoenix Games version of Peter Pan has one low-quality voice actor doing narration and characters. Figuring out who is saying what is a minigame in itself.
Chris Metzen voicing both Thrall, warchief of the Horde and Varian Wrynn, the new leader of the Alliance. They often meet in official machinima, cutscenes and lore sequences in dungeons, so Metzen ends up yelling at himself every other patch. The best example is probably the Secrets of Ulduar trailer: http://www.youtube.com/watch?v=pSbEr5ar0Zo
Blizzard generally prefers using voice changers and echos instead of hiring voice actors, so this is probably not the only case in World of Warcraft.
A common joke is: "For every male character not voiced by Chris Metzen, down a shot."
In Dragon Age: Origins, Steve Blum voices three major characters. The notable instance of Talking to Himself is when the dwarven companion Oghren is talking to a dwarven man, Gorim. The only real change in voice a bit more grit to Oghren's voice.
Mark Meer, the voice of a male Commander Shepard in Mass Effect also does voices for some of the alien NPC's, including the vorcha, Hanar and a few volus, which are regarded as the joke races. It makes the encounter with Niftu Cal (the BIOTIC GOD!) even more hilarious if Shepard is a biotic himself. Tali also brings up the vorcha similarities in the third game when she gets drunk.
Jennifer Hale plays female Commander Shepard and a minor NPC on Feros called May O'Connell, who tasks Shepard with finding new power cells to power the colony's generators.
Since there's a ton of characters in Record Of Agarest War, don't be surprised that a few seiyuus who will talk with one another. The most obvious one would have to be Leo and Rex who are both voiced by Takashi Kondo.
Rather common in the Spyro series (before the Legend of Spyro series, at least). Spyro could talk to himself when rescuing certain dragons in the original, he can talk to himself when asking the Professor for information or Hunter and Ripto can speak to themselves when facing off in Spyro 2, Bentley can talk to himself when scolding Moneybags in Spyro; Year of the Dragon, and Spyro can speak to himself in various situations in Spyro; A Hero's Tail (either having a witty banter with Hunter or a more stern banter with Red the Dragon Elder). He avoids having a conversation with Moneybags, though.
Interstate76 Nitro Pack has a mission in which Skeeter and Natty Dread do battle. They also have a few conversations over the radio. Both characters are voiced by Tom Kane.
At the end of Enslaved: Odyssey to the West, Monkey, the hero of the game voiced and motion captured by Andy Serkis, has a conversation with Pyramid, the mastermind behind the capturing and enslaving of villagers, who is also voiced and acted by Andy Serkis.
The 2009 edition of Punch-Out!! features Canadian singer Riley Inge as both Little Mac's trainer, Doc Louis, and the final boss, Mr. Sandman. Both are African-American boxers (one retired, one the champion), and strangely, both refer to Mac as "Mac, baby."
The King of Fighters has a lot of this. Just to name a few, we have Harumi Ikoma as King, Blue Mary and Yuri; Monster Maezuka as Benimaru Nikaido, Choi Bounge and Ralf Jones; and Haruna Ikezawa as Athena, Foxy and Diana.
Marc Graue voiced everyone in Hotel Mario except for Princess Peach.
Subverted in Phantom Brave: while Flonne and Marona have the same English voice actress (Sandy Fox) and the exact same voice, the scenes they share are the only ones in the entire game that aren't voiced.
Surprisingly averted in The Operative: No One Lives Forever. Kit Harris voices both Cate Archer (the main character) and Inge Wagner (one of the major villains, who is eventually a boss), but the two never actually talk to each other.
In Neverwinter Nights 2, Lisa Emery voices the githzerai cleric Zhjaeve and the succubus Blooden. It's possible to have Zhjaeve in the party when you go to Ammon Jerro's Haven, where Blooden is imprisoned in a summoning circle.
In Duel Savior Destiny Nanashi, Rubinas and Lobelia all share a voice actress and they do speak to each other quite frequently. It only makes sense: Nanashi is a homunculus Rubinas created out of her soul and Lobelia stole Rubinas' original body a thousand years ago upon the destruction of her own.
If you pair up Neneko with KOS-MOS and T-elos, Neito, T-elos, and KOS-MOS end up arguing with each other. They're all voiced by Mariko Suzuki.
In the Visual NovelBionic Heart, Dan Conlin voices Luke and Richard, Danielle Kogan voices Helen, Tanya and Tina, and Brandon Baus voices Tom and Roby. Characters voiced by the same actor have various scenes together at different points in the game.
Several examples in Baldur's Gate and its sequel. For example, in the sequel Jim Cummings voices both the ranger Minsc and several of the more memorable bosses: the dragons Firkraag and Abazigal, and the Bonus Boss Demogorgon.
In Portal 2, Nolan North voices multiple characters. You first hear him as the defective turrets, which don't interact with anyone but Chell and Wheatley. But at the end of the game, All three personality cores that you must place on Wheatley are voiced by him: The Space Sphere, The Adventure Sphere ("Rick"), and the Face Sphere. Some of their lines have them address or reference each other as you're fighting Wheatley, especially when it comes to Rick and the Fact Sphere berating the Space Sphere.
Army Men: Sarge's Heroes has exactly two voice actors. Susan Blu voices Vikki Grimm, the sole female character in the game. Everybody else - the protagonist, all five of his squadmates, his superior officer, the generic mooks when they get to talk in a cutscene, even the Big Bad General Plastro - are all Jim Cummings. Sarge's Heroes 2 adds a third actor for some roles, but otherwise keeps Blu for both Vikki and her new rival Bridgette Bleu, and Cummings for all the other named characters.
Another quite literal example happens in the Deadpool video game. In an optional scene at the beginning of the game, you can have Deadpool, voiced by Nolan North, call Nolan North himself.
Played with in the Borderlands franchise. Colleen Clinkenbeard voices both Lilith and Patricia Tannis, but they never actually talk to each other. Even in the first game, when Lilith is a Player Character and Tannis a quest giver, Lilith never responds when Tannis speaks.
In the Telltale Back to the Future game, A.J. LoCasio plays both Marty McFly and Leech (Alt!Jennifer's boyfriend); not only do they interact, but they're in active competition with each other for obvious reasons detailed in the spoiler tag.