Small multiverses in fiction.

[Millstone85]

Star Trek has exactly one parallel universe, the mirror dimension where everyone is their evil (or good) counterpart.

In the original series, possibly, but otherwise Star Trek has many timelines. Some of these coexist while others have erased each other, depending on this week's flavor of time travel / dimension hopping.

The franchise also has things akin to planes of existence, such as:

• Subspace: Primarily used for interstellar communication but sometimes said to contain lifeforms. Might also be related to "out-of-phase" technologies like the Romulan cloaking device.
• Fluidic space: A dimension entirely filled with organic matter and home to nigh-invulnerable telepathic shapeshifters called the Undine, better known by their Borg designation of Species 8472.
• Q Continuum: At once the name of a civilization of godlike entities and the higher realm in which they seem to live. The latter is perceived as a road, its true form impossible to process.

I stopped buying consoles after the SNES era, so I don't know where things went after that, but The Legend of Zelda: a Link to the Past is similarly 2-worlds.

One of the great pastimes of the TLoZ fandom is trying to figure out the branching timelines. This notably involves the MM/WW split:

Ocarina of Time	--->	Majora's Mask	--->	Twilight Princess
	--->	The Wind Waker	--->	Phantom Hourglass

And that's again before going into worlds that just exist in their own spaces, like Termina, Lorule or the Twilight.