Producers on the ABC series are currently writing and casting the characters ahead of the show's return in the fall, according to E!.
One will be a "spoiled rich girl" called Anastasia, while the other is described as "a cool badass Asian chick" named Magnolia.
It has also been suggested that Magnolia "is an accomplished fighter and rider" who could be a "bit magical".
Once Upon a Time producer Jane Espenson recently named The Little Mermaid, Goldilocks and Aladdin's Jasmine as ideal additions to the show.
Current cast member Meghan Ory (Ruby/Red Riding Hood) has been promoted to a series regular for season two, while True Blood's Kristin Bauer (Maleficent) has said she wants to return.
Once Upon a Time's first season currently airs in the UK Sundays on Channel 5.