The girls debuted (alongside Pacifica) in "Double Dipper," attending the party at the Mystery Shack along with her. They look on as Pacifica demands the party crown from Soos. They later watch the competition between her and Mabel, rooting for Pacifica the entire time, and come with her to her after-party.
In "Boyz Crazy," they are among the crowd at the Sev'ral Timez concert at the Gravity Falls Civic Center and Buffet. The two girls (along with the rest of the audience) start to fight each other over the boys' affections.
The magenta-haired one is voiced by Ariel Hirsch, who's the twin sister of the series creator, Alex Hirsch.
Boyz Crazy was their only speaking role in the entire show.
The girls cheering for Pacifica at the party at the Mystery Shack