Generally speaking, when a Yandere falls in love with someone, they tend to play for keeps. No matter how much blood has to be shed, no matter how much suffering they inflict, your common-or-garden yandere will do anything for and to be with the one they love.
So what would happen if two yanderes got together?
In theory, this would seem like an ideal set-up. With two individuals sharing such an unquestionably insane devotion to one another, you immediately bypass the typical shenanigans that accompany the archetype. Right?
Uhhhh, that's where things get interesting...
When left on their own, these couples will be Sickening Sweethearts at worst. But try to get between them, however... and you should watch out. Come hell or high water, they will do whatever it takes to remain together, and chances are that things will end badly for you and whoever stands in their way.
If this pair comes with a side of Outlaw Couple, Serial Killer, Ax-Crazy, or anything else less-than-legal, expect that little Berserk Button to become several magnitudes more difficult to avoid. If such an unholy couple is also married, this can also overlap with Unholy Matrimony (or, God forbid, The Family That Slays Together should they end up mating and producing offspring).
See also Psychotic Love Triangle, if another Yandere is butting in with the couple.
- Christine: Despite the latter being a sentient 1958 Plymouth Fury, the relationship between Arnie and the titular Christine slowly develops into this.
- Natural Born Killers: A big part of what has come to make Mickey and Mallory Knox so appealing to the media and the public is their psychotic devotion to one another.
- Suicide Squad (2016): Unlike how their relationship is depicted in the comics, Harley Quinn and the Joker are portrayed here as being utterly devoted to one another. Though by Birds of Prey, it's turned sour enough for Harley to dump him.
- Friends: Ross and Rachel serve as this, though it is much less violent. Both Ross and Rachel get jealous when they see each other with other people, even when they are not together and jump to conclusion that even when they are not together, they should not see other people.
- You (2018): Season 2 is one long deconstruction. The climax reveals that behind Love's "perfect girl" exterior, she is a cunning, remorseless murderer who killed Forty's au pair for molesting him, and she murdered Delilah and Candace for threatening to expose Joe when Joe himself couldn't. However, when she does this—and reveals she knew all along that Joe killed Beck, and planted herself to find him—Joe is repulsed by her, and only doesn't kill her because she's pregnant with their baby. They stay together, but Joe is miserable and loathes her. Season 3 carries on deconstructing it. There's more than one Hope Spot: moments when it seems like they are perfectly matched, and that this could work. But eventually it crumbles. On one hand, the ennui of married life gets to them both. On the other hand, their intensity and drama tears each other down as well.
- Vocaloid: Nem's Märchen Boyfriend and Märchen Girlfriend featuring Kagamine Rin and Len is about a mutually abusive relationship, in which both partners are constantly at each other's throats and keeping extreme tabs on the other. For example, Rin makes Len delete his female contacts and eavesdrops on his incoming calls, whereas Len makes Rin reply to his texts within 10 seconds and goes through her belongings to find a purikura of Rin with another boy. They acknowledge the relationship is destructive and borderline physically violent, but they're so attached at the hip that they can't muster the courage to split from each other.
- Umineko: When They Cry: Bernkastel and Lambdadelta are a Psycho Lesbian variant, as the entire plot, with all the gore and death, is basically one big game between them, and they frequently express their love for each other by threatening to horribly mutilate each other, like Lambdadelta threatening to gouge Bernkastel's eyes so they'll only see her.
- Head Over Heels is a messy, but realistic deconstruction of this trope, as both of the main characters seem to be in love with each other to the point it drives them to do horrific acts. However a lot of their delusions about love comes from psychological projection from their pasts, which shows that the ideals of a yandere won't stand in the real world. It's even the page image.
- Stalker x Stalker by Merryweathery features the main yanderes Junko and Yukio madly in love with each other. While fairly harmless when nobody gets between them, when somebody does get between them... well, it ends the way you expect.