Toshiyuki Morikawa and Shinichiro Miki: These two often appear together as well though it is much more apparent in BL dramas (Boys Love/Yaoi). They will usually star as major supporting characters. Some examples include Morikawa as Kurosaki Isshin (Ichigo's dad), Tousen Kaname and Orihime's Tsubaki with Miki as Urahara Kisuke in Bleach. In Naruto, Morikawa voiced Namikaze Minato and Kimimaro while Miki was Mizuki (the one who revealed to Naruto in the first episode that he is the jinchuriki of the Nine-tails). Delving into more specialized manga and anime, Miki played the Shin'Ou in Kyou Kara Maou with Morikawa as Conrart Weller. In BL, the two will usually play couples with Miki playing the younger lovey-dovey, clingy seme and Morikawa playing the older, wiser, cool beauty tsundere uke. They have partnered in: Haru wo Daiteita and its spin off Fuyu no Semi (as a couple), Sukisho (as brothers), Boku no Koe (as a couple), and Yami no Matsuei (as former partners).
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 Bishonens, 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 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 Pokémon, 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).
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 Hibarikun! 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 house guest, Kosaku. In real life, Majima (Hibari) and Furuya (Kosaku) married in 1985, and Majima subsequently retired from voice acting.
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 Generations 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.
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.
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).
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.
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.
If Soichiro Hoshi and Masayo Kurata appear in a mecha series together, it's likely that they'll play teammates, such as GUN×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).
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.
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.
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.
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. Both characters are essentially butt monkey-types in the series (and Gosunkugi in particular is a Stalker with a Crush), although Sasuke is more of the louder ham type while Gosunkugi is usually a squeaky-voiced, cowardly nerd.
High School Kimengumi: Chiba voices the buffoonish ringleader Rei, while Futamata plays Kiyoshi, the nerd of the group.
Armored Trooper VOTOMS: Although their characters don't interact, their typecasting also comes into play when Chiba plays the hammy loud comic relief pilot friend of Chirico named Vanilla. Meanwhile, Futamata was cast as Gurran, a nasal-voiced scientist who's cold and creepy but also cowardly and pathetic, leading to one or two butt monkey moments in the series.
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.
Also in the Masou Kishin subseries which stars Midorikawa (Masaki Andoh), the second PSP series introduces a serene antagonist played by Hisakawa (Elshine).
An interesting twist: Shuichi Ikeda and Keiko Han developed their relationship as Char Aznable and Lalah Sune in Mobile Suit Gundam; jump ahead 30 years, and Ikeda shows up in Hunter × Hunter as Kite, mentor to Gon...who's played by Megumi Han, Keiko's daughter. She's even said in interviews that she sees Ikeda as a father-figure.