Though not as famous as those two early examples, Hikaru Midorikawa and Nobutoshi Canna are often co-working together, as they once are under the same music group.
As noted above, in Super Robot Wars, Hikaru did Masaki Andoh, Nobutoshi did Axel Almer.
In Fushigi Yuugi, Hikaru is Tamahome and Nobutoshi is Tasuki, two Suzaku warriors.
In Persona 3, Hikaru is Akihiko Sanada, while Nobutoshi is Takaya. They're both the game's White Haired Pretty Boys, though they're on different factions.
Tomokazu Seki and Shinichiro Miki. Between the members of the Weiss Kreuz group (consisting them, Koyasu Takehito and Yuuki Hiro), they are the one who often got roles in the same series. The biggest example was Sousuke/Kurz from Full Metal Panic. However there are also other instances such as Assassin-Gilgamesh in the Visual Novel/Fate/stay night anime, whereas shortly after Assassin was killed in the anime, Gilgamesh makes his entrance (exit Miki, enter Seki). Or in the Martian Successor Nadesico. In the first series, Seki's character Gai rides the blue Aestevalis and have a very over-the-top personality. In the OVA Prince of Darkness, the one who rides the blue Aestevalis and have a rather jolly, flamboyant personality (not outright over the top, but still rather enthusiastic) is Miki's character Saburouta. (exit Seki, enter Miki).
Just a sidenote, in Pokemon, Seki was Kenji (Tracey) and Miki was Kojiro (James). Their encounters were mostly in the Orange Islands (so its enter Seki, exit Seki, with Miki staying still).
Rival band vocalists Shuiichi Shindou of Bad Luck and Taki Aizawa of Ask in Gravitation
Rival racers Takumi Fujiwara and Keisuke Takahashi in Initial D, along with Koyasu Takehito for the ride
Bob Makihara and Masataka Takayanagi in Tenjho Tenge
Minamoto no Yorihisa (Miki) and Tenma Morimura (Seki), the Seiryuu Hachiyou in Harukanaru Toki No Naka De.
In Hakuouki's prequel, Reimeiroku, Miki played the role of Hijikata while Seki as the main chracter, Ibuki Ryuunosuke.
Tomokazu Seki has an alleged rivalry with Tomokazu Sugita. Their characters often have strong interactions with each other as well: Sugita's Yuuichi Aizawa from Kanon, Kagami Mikage from Ayashi No Ceres, and Hideki Motosuwa from Chobits, to Seki's Jun Kitagawa, Alex Howell and Hiromu Shinbo, respectively. Lampshaded in an episode of Lucky Star, in which the character Sugita voices works in the same anime shop as Meito Anisawa/Anime Tenchou, Seki's role.
This even extends to video games, if one pairs up Christie and Kasumi in the Dead Or Alive series.
The 1983 TV series Stop! Hibari-kun! featured voice actors Satomi Majima and Tohru Furuya as the two central characters, Hibari and Kosaku. In this series, Hibari, a yakuza boss's son who is living life as and posing as a girl, develops a crush on his houseguest, Kosaku. In real life, Majima (Hibari) and Furuya (Kosaku) married in 1985, and Majima subsequently retired from voice acting.
In Naruto Ishida plays Gaara and Hoshi plays Yashamaru, his uncle and the only person he ever loved. They also play friends Hakkai and Goku in Saiyuki.
Soichiro Hoshi and Kenichi Suzumura. Being in the same company, they have done twins (in the early Ouran drama CDs) and bitter rivals (Gundam SEED Destiny's Kira/Shinn). And in Kara No Kyoukai where Hoshi played a sadistic serial killer while Suzumura played a guy on said killer's hit list!
Takahiro Sakurai and Kenichi Suzumura are known best friends in real life, even going so far as having their own radio station. They usually appear together in a lot of things, especially BL audio dramas, but none so infamous as them playing opposites in the Final Fantasy VII compilation as Cloud Strife and Zack Fair, respectfully. They also form the singing duo R-16.
It gets funnier if you connect Tales Of Hearts and Hell Girl. Kaori Shimizu (who plays Richia Spodune in Tales Of Hearts) gets to guest star in the show as a girl who is eventually sent to Hell, whereas she is carried to hell in the ferry by Enma Ai (Mamiko Noto) and Ichimoku Ren, who is played by the same guy who did Hisui Hearts in Tales of Hearts, Masaya Matsukaze.
Another interesting note in Kaori Shimizu's relationship is with Yumi Kakazu. There are two shows that makes a different parallel to their roles: Super Robot Wars and Godannar. One of them will play a senior lady in war (Lamia (Kaori) in SRW, Shizuru (Yumi) in Godannar) probably one with large breasts, while the other one will play a younger, junior pilot compared to the other (Seolla (Yumi) in SRW, Lou (Kaori) in Godannar). Thing is, these 'senior' ladies will appear to get killed in the middle of the show, only to be brought back later. Lamia 'dies' in Super Robot Wars Original Generation bonus section, and in OG Gaiden, she came back Brainwashed and Crazy and eventually restored. Shizuru is killed off in the end of Godannar Season 1, and Season 2's first episode deals with her resurrection.
Ryotaro Okiayu and Tetsu Inada. They like to play as allies, or sometimes closer than that. No, not Yaoi stuffs. Let's see...
In the crossover of Dynasty Warriors and Samurai Warriors, Warriors Orochi. Orochi is voiced by Okiayu, whereas one of his allies, Lu Bu, is voiced by Inada.
In Bleach, Okiayu is Kuchiki Byakuya, Captain of the 6th Division, while Inada is Sajin Komamura, Captain of the 7th Division (Okiayu's squad number plus 1 = Inada's squad number).
In the Super Robot WarsOriginal Generation, Okiayu voices Raidese F. Branstein, while Inada voices his Aloof Big Brother Elzam von Branstein. In OG Saga', Inada is Ezel Granada of the Orchestral Army, whereas Okiayu is... Ezel's ex-right hand man Katze Kotolonos and Ezel's war-rival Rubor Cucullus. In Masou Kishin'', Okiayu and Inada end up voicing both of Tytti's familiars Freki and Geri.
In Castlevania (especially Dawn of Sorrow), Okiayu is Genya Arikado (a.k.a. Alucard), Inada is Julius Belmont. And they play as the Obi Wan of the hero Soma Cruz... or can team up in one of the outcomes of the bad ending.
Speaking of Morita, he is also often cast with Junichi Suwabe. They're either pairs Auel (Morita) and Sting (Suwabe) in SEED Destiny or rivals Ichigo (Morita) and Grimmjow (Suwabe) in Bleach, Tidus (Morita) and Seymour (Suwabe) in Final Fantasy X.
This doesn't stop fans from pairing their SEED Destiny characters, though, despite the fact they never even met on screen.
It's also rather fun to note that Jun Fukuyama and Norio Wakamoto are Lelouch and Emperor Charles respectively in Code Geass, while previously in Tales Of Destiny 2, they're also enemies (Fukuyama is Kyle, Wakamoto is Barbatos). Also noted that Fukuyama's side is usually backed by a character played by Hikaru Midorikawa (Li Xingke and Judas, respectively).
A case of working together between Fukuyama and Midorikawa (minus Wakamoto) can also be seen in Gravion, whereas Fukuyama is Touga, while Midorikawa is Raven.
While we're on the subject of Wakamoto Norio, expect George Nakata not to be too far behind. For example, in Hellsing Ultimate, Nakata voices Alucard, and Wakamoto voices Anderson. Also, not so much of a relation, but Nakata voiced Dracula in Castlevania Judgment, replacing Wakamoto, his usual voice.
Toshiyuki Morikawa and Nozomu Sasaki are siblings in real life, and if they're in the same series they usually play brothers (the Frost brothers in Gundam X, with Morikawa as Shagia and Sasaki as Olba) or best friends (Morikawa was Takashi and Sasaki was Youta in the Video Girl Ai CD dramas)
Saeko Chiba and Yuuka Nanri are good friends and even had a duo for a time. Chances are, if they're in the same series, they'll play either sisters (like the Iijyuin twins in Prince of Tennis) or rivals (like Natsuki and Nao in Mai-HiME).
They also worked together in After War Gundam X as Shagia Frost (above) and (this is fairly odd) Caris Nautilus.
Some of the Rival Schools cast is actually a revisit from GaoGaiGar. Nobuyuki Hiyama is Guy and Batsu and Tomoe Hanba is Mikoto and Hinata (and they reprise their love interest relationship in GGG as well). Miki Narahashi plays an Eagleland character in both series (Swan and Tiffany) as well.
Gundam 00 revisits and reverses the Kingdom Hearts main cast. Sora is now pissed off at Riku, who got promoted from The Rival to The Hero (sort of). Yes, they got the same voices with Saji Crossroad and Setsuna F. Seiei, respectively. (Though Saji did became pissed at Setsuna ( in season 2), it's not because of the latter's character position).
More evident starting episode 11 of season 2, when Saji and Setsuna co-pilot the eponymous mobile suit.
Keiji Fujiwara voiced red haired baddies for both series, namely Ali Al-Saachez and Axel.
And it doesn't help that Masaya Matsukaze is respectively Kyouka Ohtori and Teru Mikami.
Later on, the former two would be working on Soul Eater as Death the Kid and Crona respectively (oddly enough many ship them as a Crack Pairing of sorts).
And finally they are going to work together in Star Driver
Love Hina is even more enjoyable when you remember that Megumi Hayashibara and Yasunori Matsumoto, the voice actors for Haruka and Seta, also played Lina and Gourry in Slayers.
Don't forget that they're also Aki and Noal in Tekkaman Blade.
And Crysta and Zak in the Japanese version of Ferngully.
Hitomi Nabatame has a variety of relationships with voice actors, she and Kenichi Suzumura have done Drama CD/Anime of H-game OVAs together which often portrays them as a couple. Although she is more noted for her work with Shizuka Itoh and Mamiko Noto. She also works with Yui Sakakibara in in a lot of Eroge (non-yuri of course).
From Rika Furude and Rena Ryuugu to Nanoha Takamachi and Teana Lanster to Mei Sunohara and Nagisa Furukawa to Midori and Mai Tokiha to Togame and Nanami Yasuri to Shuri and Asami is a long way to go indeed.
Making it two out of five, Satsuki Yukino voices Shmion and Misae Sagara.
Saki, whether intentional or not, seems to enjoy using this, as Touka, voiced by Minori Chihara, has a butler, Hagiyoshi, voiced by Daisuke Ono. These two played Nagato and Koizumi in Haruhi Suzumiya.
Kikuko Inoue: Akane Sonozaki in Higurashi, Virgilia in Umineko
Jurota Kosugi: Okonogi in Higurashi, Krauss Ushiromiya(and Okonogi again) in Umineko
Yuu Kobayashi: Satoshi Hojo in Higurashi, Kanon in Umineko
In addition to the Higurashi and Umineko examples above, Yui Horie and Yukari Tamura have co-starred in a number of other shows, oftentimes as competing haremettes/love interests of the male lead. Examples include Ayu and Mai, Yasuna and Tomari, and Kotori and Sakura. (They've also temporarily formed a singing duo in Real Life.)
Kikuko Inoue and Soichiro Hoshi did a bit of relationship voice acting where their characters are very close to one another at some point, Kikuo voiced Kira's foster mother in Gundam SEED, Please Teacher! as Kei's spouse and being Grace's puppet in Macross Frontier being the only exception.
If Soichiro Hoshi and Masayo Kurata appear in a mecha series together, it's likely that they'll play teammates, such as Gun X Sword (Hoshi as Michael and Kurata as Fasalina), Gundam SEED (Hoshi as Kira and Kurata as Mayura), Muv-Luv (Hoshi as Takeru and Kurata as Chizuru) and late into Code Geass (Hoshi as Gino and Kurata as Rakshata).
Jun Fukuyama and Sayaka Ohara. Most of the time, Fukuyama is Ohara's underling/assistant, but this is sometimes reversed.
Gundam SEED Stargazer: Selene MacGriff (Ohara) with her assistant Sol Ryuune Lange (Fukuyama)
Xxx Holic: Yuko Ichihara (Ohara) with her bitch Watanuki (Fukuyama)
Code Geass: Subverted, somewhat. Milly Ashford (Ohara) with her... classmate in-class Lelouch (Fukuyama). However, in the bigger picture... she amounts to nearly nothing on Lelouch's grander scheme.
Valkyria Chronicles: Inverted. Selvaria Bles (Ohara) with her boss Maxmillian (Fukuyama).
Ayumi Fujimura and Nobuhiko Okamoto have a non-existent relationship in Hatsukoi Limited even if they're both there, with the former being the artistic and shy Chikura Nao and the latter the boyfriend of Dobashi Rika. Interestingly, Mariya Ise also shows up there as Arihara Ayumi.
Gets funner when Tomokazu uses this as an excuse to invite Mikako to guest star the Bururaji radio, with Heart Aino used as Mikako's avatar. And mind you, during a previous April Fools joke, there was a shot for crossover between Arcana Heart and BlazBlue, with Heart (Mikako) being paired with Ragna (Tomokazu).
The SRW relations between Sugita and Nakamura expands now, with Brooklyn 'Bullet' Luckfield (Sugita) and Joshua Radcliff (Nakamura). Though thus far, they're not so related aside of being in the same side (eventually).
Gintoki Sakata and Kintoki Sakata in Gintama. Practically the best Casting Gag that Sunrise pulled off considering they're best friends in real life.
Daisuke Ono and Sayaka Ohara often plays the role of a bickering couple with Sven and Selene as well as Battler and Beatrice where their bickering tends to drag into the lethal side
A triang relationship can be seen within Katsuyuki Konishi, Tomokazu Sugita and Tetsuya Kakihara. There are three series in which involves only two of them (one will be missing) and these characters will be deeply connected.
Maaya Sakamoto and Joji Nakata have had two instances of being enemies in a fight to the death, though one's a Blood Knight protagonist, and the other's a one-episode antagonist. In the fifth Kara No Kyoukai movie, Sakamoto played Blood Knight protagonist Ryougi Shiki, and Nakata played one-shot villian Souren Araya. In the fourth episode of Hellsing Ultimate, the roles were reversed, with Nakata playing Blood Knight main character Alucard, and Sakamoto playing one-shot villian Rip Van Winkle.
Asami Imai and Chiaki Takahashi has been in several series together ever since their time together in the original The Idolmaster (Chihaya and Azusa), which includes being in Koihime†Musou (Kakuka/Jia Xu and Choukun/Zhang Xun), BlazBlue (Tsubaki and Litchi) and Neptunia (Noire and Magiquone).
Also though not really direct, Chiaki appeared in Chaos;Head. When the Spiritual SuccessorSteins;Gate appeared, she's not there, but Asami appears there (being Kurisu Makise)
In general, Idolmaster seiyuus usually form strong bond together. Eriko Nakamura has been a great friend with Asami Imai, also appearing (along with Imagine and Chiaking) in Koihime†Musou as Enjutsu/Yuan Shu, along with her roles as Ui in Hoshizora e Kakaru Hashi (with Chiaking as Tsumugi), and as Chisato in Koi To Senkyo To Chocolate (with Imagine as Michiru). On the other hand, Chiaking also once worked together with Mayako Nigo (Yayoi) in Omamori Himari, and the former had a minor role in Zoids Genesis, which stars Hiromi Hirata (Makoto). Likewise, Hiromi Hirata and Mayako Nigo can be spotted together in Arcana Heart (as Kamui Tokinomiya and Lilica Felchenrow respectively)... together with Juri Takita (Kotori, in Arcana Heart as both Mei-Fang and Mei-Ling)... who was in the Japanese version of Fire Emblem Path of Radiance and Radiant Dawn as Princess Elincia, along with Akiko Hasegawa (Miki) as Mia/Wayu.
And then again when Rusty Hearts, a Korean MMORPG gets announced for a Japanese release. They first got Hiromi Hirata doing Natasha Borzenkova, and when the Avatar characters are released, they bring in Chiaki Takahashi as Roselle Vergerius and Asami Shimoda (Ami/Mami) as Chen Meilin.
Kana Ueda and Shizuka Itou have played alongside each other multiple times, often with Itou's character being Ueda's superior such as Kohaku and Akiha, Yumi and Rei, and Saki and Hisa. Lyrical Nanoha flips this around with Ueda's character, Hayate being of higher rank than Shario, who is voiced by Itou, and Fate/unlimited codes has Ueda and Itou voicing rivals Rin Tohsaka and Luviagelitta Edelfelt respectively.
Akira Ishida and Mamiko Noto have appeared in a lot of shows. We've got Mnemosyne (Apos and Rin), Persona3 (Main protagonist/Pharos/Ryoji Mochizuki and Fuuka Yamagishi), Keroro Gunsou (Saburo and Angol Mois), Negima! (Fate Averrncus and Nodoka Miyazaki), Queen's Blade (Delmore and Tomoe), and Mawaru-Penguindrum (Keiju Tabuki and Yuri Tokikago).
Daisuke Ono and Miyuki Sawashiro have twice been Platonic Life Partners: as Shizuo and Celty in Durarara and as Huey and Dalian in Bibliotheca Mystica de Dantalian. Interesting to note is that Sawashiro's characters in both shows are Tsunderes of varying degrees while Ono's characters in the shows are usually at opposite ends of the emotional spectrum.
Shigeru Chiba and Issei Futamata have been cast in various series together throughout their careers. When their characters actually interact, Chiba tends to play the louder and dramatic one while Futamata's part is more of the Butt Monkey in comparison.
Maison Ikkoku: Futamata is Yusaku Godai, while Chiba plays the strange neighbor Yotsuya.
Urusei Yatsura: Chiba voices Megane, while Futamata voices his underling Chibi.
Ranma ½: Although their characters never get to interact, Chiba voices Sasuke Sarugakure, who replaced the character of Hikaru Gosunkugi in the early seasons of the anime. When Gosunkugi finally showed up, Futamata was cast in the part.
High School Kimengumi: Chiba voices the buffoonish ringleader Rei, while Futamata plays Kiyoshi, the nerd of the group.
In Sailor Moon, Hisakawa played Ami Mizuno/Sailor Mercury, while Midorikawa played both Ail (one-time main enemy of an arc), and Fiore (movie villain)
In Heartcatch Pretty Cure, Hisakawa played Yuri Tsukikage/Cure Moonlight, while Midorikawa played Dune (Big Bad and responsible much of Yuri's sufferings)
In Kessen III, Midorikawa reprises his role as Akechi Mitsuhide, who now plays as the villain who betrays the now-sympathetic Oda Nobunaga, and kind of lusting for Kichou/Nouhime, played by Hisakawa.
