If two girls sleep with one of the girls’ father, is it considered yichud?
If a daughter is present, there is no prohibition of yichud with another girl.
Sources: See Devar Halachah (8:4), who quotes dissenting opinions concerning this question. Rav Wosner, however, is lenient on the matter, and this leniency is also cited in the name of Chazon Ish (see Mibeis Levi, vol. 4, p. 48). This is all the more true when the other girl (not the daughter) is not a married woman.