Exarchopoulos/Seydoux or Tilly/Gershon

Adèle Exarchopoulos and Léa Seydoux in Blue is the Warmest Color (2013), perhaps the best girl-on-girl action in the history of mainstream films.

One of the competitors for that honor, perhaps the former champion, involves Jennifer Tilly and Gina Gershon in Bound (1996)