Mutually Fictional

This is a special kind of crossover trope, where the characters from Show A will enter the universe of Show B--both shows of which are "real" to us. In other words, neither is a Show Within a Show. In addition to finding out that they're trapped in the universe of Show B, the characters of Show A discover that they themselves are the subject of a Show A in the universe of Show B. The characters from Show A are, in essence, simultaneously Trapped in TV Land and a Refugee From TV Land. This isn't a Real World Episode, since both universes are depicted as being equally "real".

A situation in which Show A is fictional in Show B and Show B is fictional in Show A isn't an example of this trope if they never share a continuity; if in A's continuity B is just fiction and vice versa. This could happen with two completely unrelated works that each incorporate real world elements that happen to include the other work.

This is a relatively common trope used in Crossover Fanfic.

Strictly speaking, this kind of crossover should never logically be allowed to exist. At the very least, the particular episode of each series or work which references the other should be assumed to not exist within the other's universe. Otherwise, you would have a situation wherein it would be distinctly possible for the main characters to see the TV show of their entire reality within said reality, realize their entire existence was a lie, and freak out. And we wouldn't want that, now would we?

One possible justification would be if the two worlds are simply Alternate Universes and the "shows" in question are based on visions people have from the other world. In this case, expect the characters trying to establish what in this shows is correct and what is not. Unfortunately, the Fiction Identity Postulate proves that all fiction is equally unreal. And anyone living in an Alternate Universe may be, by definition, fictional.

This is where Recursive Canon meets Recursive Reality. May create an accidental Intercontinuity Crossover.

See also Celebrity Paradox. Comic Books Are Real is a one-sided version, usually dealing with a Show Within a Show instead of another real-life series. Compare Faeries Don't Believe in Humans, Either, where each side believes the other is only stories prior to meeting, but both have always been fact and that's what the stories are based on. Contrast Stable Time Loop, which leads to a similar Ontological Paradox.

No real life examples, please; at least, not until we have proof that somebody from another universe can interact with somebody from this one.

Examples of Mutually Fictional include:

Comic Books

Once upon a time, Milestone Comics and DC Comics' Superman books participated in a Crisis Crossover, Worlds Collide. The Blood Syndicate (essentially a streetgang with powers) were the first Milestone characters to meet Superman, and although they thought he was just a local wannabe, they all immediately knew who Superman was, what he could do, etc. because Superman was a comic book character in the Milestone 'verse. ("Does your mama know you left the house looking like Clark Kent?!"). Superman doesn't have the same benefits, realistically, since the Milestone Comics characters were hardly a household name, and he's not much of a comic fanboy.
- Static, an Ascended Fanboy, lampshades this; he drops his knowledge of Post-Crisis Superboy's history, and explains "I read all your comic books! Don't you read all of my comic books? (Do I have comic books?)"
- In the wake of a Cosmic Retcon, the two universes have now been merged with a new, shared history. Only a handful of people (including Superman) remember that they were ever separate.
This happens to Superman a lot; it used to be that DC/Marvel crossovers operated under the conceit that the characters, if they didn't know of each other, at least operated in the same reality for the duration of the Crossover (Spider-man/Batman, for example), but after DC vs. Marvel/Marvel vs. DC, they were explicitly separate realities. It is true that the Fantastic Four knew of Superman from the events of that crossover in Superman/Fantastic Four, it was also established that Ben Grimm and Franklin Richards knew of Superman from the exploits of his comic book counterpart.
- Incidentally, Marvel vs. DC played with a retcon of Spider-man/Batman when the Joker recognized Spider-man from somewhere. Of course, since S/B was set before the Spider-clone saga and DC vs. Marvel was set after, Ben Reilly Spider-man didn't recognize the Joker from Peter Parker's adventure.
Speaking of DC Comics, Pre Crisis at least, Earth-One and Earth-Two were fictional to each other, and on Earth-Prime, supposedly all the other alternate universes were fictional.
The first Futurama/Simpsons Bongo Comics crossover comes about as the Brainspawn zapped the Planet Express crew into an old Simpsons comic. The second has the Simpsons characters and later, many other fictional characters materialising into the reality of Futurama from a comic by one of Farnsworth's inventions. However in their Bongo Comics crossovers, The Simpsons are pointed out as the fictional ones in Futurama's "real" universe. It should be only one way, being built off the idea in "The Day the Earth Stood Stupid" where the Brain can take people into worlds of fiction but since Matt Groening cameos on The Simpsons as the creator of Futurama...
She Hulk's second series (where she can ignore the Fourth Wall has always been Loose Canon as it is; in her third series, it is revealed (via Retcon) that the second series is a fictional account of her life that is published in Earth-616, as in mainstream Marvel Universe.

Fan Works

Fanfic author Jared "Skysaber" Ornstead used this trope to invert the Self-Insert Fic trope of Author Avatars knowing everything about the worlds they visit; there's always a Show Within a Show based on his life in each one, and at least one of the characters is guaranteed to be a fan.
In the Harry Potter and Fate/stay night crossover Fanfic "Fictional", Harry is a servant created by Caster from the book series. A big part of the plot is Harry coming to terms that all of his hardships were fictional and how to deal with it after the obligatory freak out. And you know, deal with being a slave (*cough* Servant). Did I mention that he also has to hide his scar, because other people freak out when they meet Harry Potter too?

Film

The Fountain is this.

Literature

From Through the Looking-Glass:

"What ... is ... this?" he said at last.
"This is a child!" Haigha replied eagerly, coming in front of Alice to introduce her, and spreading out both his hands towards her in an Anglo-Saxon attitude. "We only found it to-day. It's as large as life, and twice as natural!"
"I always thought they were fabulous monsters!" said the Unicorn. "Is it alive?"
"It can talk," said Haigha, solemnly.
The Unicorn looked dreamily at Alice, and said "Talk, child."
Alice could not help her lips curing up into a smile as she began: "Do you know, I always thought Unicorns were fabulous monsters, too! I never saw one alive before!"
"Well, now that we have seen each other," said the Unicorn, " if you'll believe in me, I"ll believe in you. Is that a bargain?"
"Yes, if you like," said Alice.

Robert A. Heinlein's The Number of the Beast introduces the concept of the "World as Myth" which supposes that all fictional universes are equally real and, moreover, are accessible to one another via interdimensional travel. The act of authorship is what creates said universes, which leads to the interesting notion that the characters in any given universe may be controlled, at any given moment, by an Author from another. Or that characters could, in theory, meet their own Author. The novel concludes in a Massively Multiplayer Crossover whereby the protagonists host a convention of characters from nearly every Science Fiction and Fantasy universe ever.
- The subsequent novels The Cat Who Walks Through Walls and To Sail Beyond the Sunset run with the concept to its logical conclusion, in which the characters wage running pandimensional battles against groups of agents from other realities, all competing to see which can rewrite history to their whims.
Philip K. Dick's The Man in the High Castle contains a rather odd example. The novel is an Alternate History describing a hypothetical timeline in which the Axis won WWII and conquered the United States. And in the novel's 'verse exists another novel The Grasshopper Lies Heavy, which is itself an Alternate History novel describing a hypothetical timeline in which the Allies won WWII.

Live-Action TV

Power Rangers in Space did some sort of Mind Screw in the crossover episode with the Teenage Mutant Ninja Turtles; given the context of the episode, they seem to exist in a Shared Universe, but each team believes the other team to be fictional. Until they actually meet, that is.

Web Comics

There's a one-off joke in El Goonish Shive when it turns out that the Author Avatar Dan, is actually a fictitious character created by Sarah.

Western Animation

In Futurama, "The Farnsworth Parabox," the Professor A creates a box containing Universe B (or, "the Flying Mongooses") at the same time the Professor B creates a box containing Universe A.