I think you've already taken some great steps! Perhaps you could take a week to promote friendship/kindness! There are a lot of really great children's books that you could incorporate. You could do a week long group project where the two are actually paired with someone else! It could be a project where they trace each other's bodies using a large roll of paper. They can help color it and then (all with lots of hep) write kind words about their friends on the cut out of their friend. Make their shirt and pants have kind words! It could create a bond...then you can group all the girls together on the wall!! Reiterate that they can ALL be friends, point out the sweet smiles and kind words!
Just a thought!
The September 2019 issue of Young Children contained an excellent article; Two's Company, Three's a Crowd: Peer Interactions in a Preschool Social Triangle (Voices) by Christopher Robert Taaffe. Though it deals with triad groups I believe the information pertains to this situation as well.I agree with Aren Stone's comment. The children need to be taught how to integrate others into their play.