The Edelfelt sisters were considered one Master in the Third Holy Grail, but their unique Sorcery Trait allowed them to summon two sides of the same Heroic Spirit. The Sabers were together for a time, but later served separately under their respective Masters after they split apart. While the sisters were said to be defeated early on, the Saber-class has a reputation of making it to the final battle of the war. The exact details are unknown. The Holy Grail War ended without a victor due to the destruction of the Lesser Grail's vessel. One is said to have been killed, while the other's whereabouts became unknown.
They appear in Fate/hollow ataraxia under the guise of Rin Tohsaka and Sakura Matou due to Avenger being unable to accurately recreate them. Their Servants take on the appearance of Saber and her corrupted self, Saber Alter, and they quickly dispatch Bazett Fraga McRemitz and Avenger during the first loop while they are out on reconnaissance.
They possess the Ore Scales Sorcery Trait, which allows them to share the family's Magic Crest and lineage. It allows them to summon two aspects of their Servant while still remaining as a single Master.