Mutual Masquerade

Due to the very nature of The Masquerade, one of the toughest things about it is finding other people who are also in on the Masquerade. This is justified since it wouldn't be very good form to go around asking everyone you meet if they know that there's a secret society of aliens or magical creatures living among normal people. If everyone does their job right, everyone in on the masquerade should appear like ordinary Muggles ... even to each other.

Of course, this type of situation could lead to many surprises. Suppose you're a mystical creature living in a world where mystical creatures are thought not to exist, and you must maintain the persona of being an ordinary person. You know that there are others like you that exist, but other than your close friends and family, you don't know who or where they are. You try your best to maintain your disguise, lying to all the muggles at school and at your job. But one of them is getting incredibly close to your secret. You've known this person for years, and never thought him to be anything more than an ordinary muggle. But when he discovers your secret, it's actually okay, because he has the same secret himself!

In Fantasy Kitchen Sink situations (or Crossover) they might have a completely different secret. If not a directly opposed faction; it's usually not a problem; as it is easy to agree to "not reveal your secret if you don't reveal mine."

Of course, it could work the other way around too. The trope is invoked whenever a character maintaining the masquerade discovers that another person, whom they've known for awhile to be an ordinary muggle, is revealed to them to also be a part of the Masquerade. Whether the second person has already known of the first person's involvement in the masquerade is irrelevant.

Related to Right Hand Versus Left Hand. In extreme cases can result in a Flock of Wolves.

Anime & Manga

 * In Mahou Sensei Negima, Negi and some of his students find out that each other knows about The Masquerade.
 * In Darker than Black this is an important part. In the very first episode a scientist on the run explained basics about what's going on to a nice lad who just happened to be around and help her. Guess what, he was much better informed. Then, of course, Contractors can't identify each other (there's no way to tell right until one actually does something a normal human can't or sees what normal humans don't). And no one, including multiple intelligence services and Syndicate's own agents below top-tier, knows for sure extend of the Syndicate's influence, or how this or that organization is connected to it, let alone who exactly. Especially seeing how it incorporated many sub-factions unwilling to miss an   opportunity for mugging each other unless high-ups yank the chain.

Comics

 * There's a Golden Age DC Comics story about the first meeting between Clark Kent and Bruce Wayne - neither of whom have any idea the other one is also a superhero. (They find out before the end of the story, when they both see the other changing into costume.)
 * In DC Vs Marvel, Clark is working with the Planet's new photographer, Peter Parker, leading to a scene where they both try to make excuses to the other in order to change.
 * Cassidy meeting another vampire in Preacher (Comic Book).

Film

 * In the film Batman Returns, Bruce Wayne is dating Selina Kyle; he doesn't know she is Catwoman, and she doesn't know he is Batman.
 * Mr. and Mrs. Smith, the movie where husband and wife discovered each other's secrets: They were both assassins for hire.

Literature

 * The Man Who Was Thursday: The protagonist is recruited as a secret agent so secret that none of the agents know who any of the other agents are. There are several moments later in the book where he discovers that some person he was suspicious of is actually one of his colleagues (and was, equally unknowing, just as suspicious of him).
 * Louis' first introduction to Armand in Interview with the Vampire.
 * Fairly common occurrence in the Deryni works set after the beginning of the persecutions, since many Deryni went into hiding one way or another to save their lives. A notable example occurs in The King's Justice, when Bishop Arilan finds Jehana guilt-stricken after she warned Nigel of an assassination plot, knowledge she acquired thanks to her powers. First he tries to teach her through a couple of parables that she had an affirmative duty to reveal what she had learned, and when she retorts that he doesn't understand, he displays his Deryni aura to show her that he actually does.
 * Harry Potter: Despite having known about wizards and himself being a wizard for years, he was still surprised to find that Mrs. Figg, one of his Privet Drive neighbors that he's known his entire life, has been in on the Masquerade the whole time and has also been tasked by Dumbledore to keep him safe. He tries to hide his wizardry (he had his wand out to cast his Patronus against the Dementors that attacked him and Dudley), and she urges him not to put his wand away.
 * L.J. Smith's Night World series had a book called "Secret Vampire" that centered around vampire James Rasmussen and his mortal girlfriend Poppy. Poppy is terminally ill, so James reveals himself to her as a vampire and turns her in order to save her life. James has broken the second of two major laws in the Night World: Never tell a mortal about the Night World. (The first is "Never fall in love with a mortal" and he'd already broken that one.) At the end of the book, however, it turns out that, unbeknownst to both him and Poppy,
 * The Wheel Of Time series has the Black Ajah working in this fashion. In order to prevent any Black Sister from having the knowledge needed to betray the entire organization all Black Sisters except the top few are normally only ever given the identity of three other members. Most members do not even know who the next person up the chain of command is, much less who is at the head.
 * In the novel Twelve, vampires have to hide themselves from human society. The protagonist discovers that the "mercenaries" his friend hired are in fact vampires.

Live-Action TV

 * The Batman series from the 1960s included a pair of cross-over episodes with the Green Hornet series. Bruce Wayne and Britt Reid originally met and socialized (they are both millionaire playboys, after all), with neither aware of the other's identity as a masked crimefighter. The villain of the episode actually figures out that the two billionaires are also the two heroes but mixes up the secret identities (thinking Wayne is Green Hornet and Reid is Batman). When Batman proves this wrong the idea is completely dismissed and the truth never occurs to anyone, including the heroes.
 * One episode of Amazing Stories featured the tale of a high-school-age boy whose parents revealed to him that a) they were actually alien scientists who came to Earth to study the "primitive culture" and b) their mission was over so they were returning home. The boy regretfully says goodbye to his High School sweetheart, lying to her about his dad getting a new job in a new city and them all moving away. On the spaceship, just before launch, he finds his girlfriend. Turns out her parents were also alien scientists, and that they are all going home.
 * In Forever Knight, Nick's partner Tracy knows that vampires exist and knows that Vachon is a vampire. However, Vachon and Nick hide the fact that Nick is a vampire from her, and she doesn't find out until the last episode.
 * Buffy the Vampire Slayer season 4- The Initiative are vaguely aware of the Slayer, but have no idea she a) really exists and b) is Buffy, while Buffy has no idea that her new boyfriend and college professor are part of a military operation to do what destiny has decided is supposed to be her job.
 * On Heroes a only relatively few people with abilities know that anybody else has abilities.
 * Wizards of Waverly Place: There have been several characters the Russos have met and have known to be muggles to them for quite a while before it was discovered that they were wizards or magical creatures as well. Notable examples are Stevie, Mason, and TJ.

Tabletop Game

 * Common occurrence in New World of Darkness campaigns, due in part to the sheer number of masquerades that know next to nothing about each other.
 * Varies. When you're a vampire, werewolf, mage or changeling, you can expect to be inducted to your respective secret society, in which, generally, everyone knows who is who. Thus it's a rare occurence that a vampire is in town without the other vampires knowing (and this is the way things should be—masquerade breach is deadly for the vampires as a whole).
 * Meanwhile, the same luxury can't always be afforded by hunters, the Frankenstein monsters, and sin-eaters. Sometimes you don't even know that there are other hunters in your area. And one doesn't need another sin-eater to become a sin-eater, so it's likely that a sin-eater might go a long way without meeting the others.

Webcomic

 * In El Goonish Shive, quoth one high-ranked member of The Men in Black, "the accessibility of magic has always been the real secret". So there's a lot of magic users who have no clue about each other and lose time in otherwise trivial cases. Occasionally involving (if Noah is right) deaths that could be easily avoided and manage to manipulate whole families into hiding it from each other behind white lies, like Nanase and her mother (who they know are both well aware of magic) to keep it in secret from each other "for now"... which later would be easier to keep than admit; even the "good guy" Mr. Verres let his own son turn into complete wreck thinking his mother (steering clear because she is involved in too much of dangerous action) left without trace because she doesn't love him, and even after  convinced his niece to keep a piece of this puzzle to herself.
 * Eventually, The Men in Black coercing or tricking the rest to stumble blindly proved highly counterproductive to the whole thing. A wizard's apprentice Noah was trying for days to investigate a magical incident caught by a TV crew (if not very well), and since he couldn't tell what it's about and had zero diplomatic skills, he was forced to go the roundabout way through Elliot without telling him anything. Elliot himself worked with another investigator connected to The Men in Black, being a magic-user and knowing about the wizard but not Noah. By the time he got to the witness (yet another magic user who by this time had a good idea what's going on, but wasn't going to tell Noah anyway), the delay allowed.

Western Animation

 * The Fairly OddParents: Timmy Turner has been classmates with a rich kid named Remy. After a series of suspicious events, Timmy has decided to spy on Remy and discovered that he also had a fairy godparent. When Timmy asked his godparents why he was never told, the fairies replied that it was against Da Rules for faeries to tell their godchildren about other children with faeries. They have to figure it out on their own.
 * The Batman: As in Forever, Bruce and Selina are attracted to each other in and out of costume.
 * Zorro: Generation Z: Diego and Maria have no idea of their counterparts being Zorro and the Scarlet Whip,