"Tohru and Legend", which deals with Tohru's past before she came to Earth.
"Ilulu and Part-Time Job" which focuses on Ilulu starting to work at Aida's candy store and meeting Taketo for the first time.
Miss Kobayashi's Dragon Maid has several:.
As the title states, this "duel" was extremely heartbreaking.
One Piece had the first (and so far only) named two-parter episode called "A Heartbreaking Duel: Luffy vs.
Kaguya-sama: Love is War has several of these, though the most iconic would be the two-parter "I Can't Hear The Fireworks", which ended up causing a shift in the status quo regarding Shirogane and Kaguya's Battle of Wits.
The first 19 episodes of the show alone take up a single over-breaching arc, followed by episodes 40-50, and then almost the rest of the series afterwards is one long arc that takes up almost half the series (aside from episode 90, which is a filler gag episode). This is mostly due to adapting stories of different lengths (with Melancholy and Sigh being entire books) into a series with 30-minute episodes.
The Melancholy of Haruhi Suzumiya has the 6-part "The Melancholy of Haruhi Suzumiya", two-part "Remote Island Syndrome", 8-part "Endless Eight", and five-part "The Sigh of Haruhi Suzumiya".
One, "The Most Dangerous Race", was a three-parter.
Speed Racer was mostly made up of two-parters during its run.
Magical Girl Lyrical Nanoha StrikerS contained two two-parter episodes, one introducing the seasons' Living MacGuffin and one where she is kidnapped, setting up the Final Battle.
Magical Girl Lyrical Nanoha A's had a two-parter explaining the season's Anti Villains' origin story.