I'm with Ember Rose. I'd most likely try to plan play dates so that either one girl is at a friend house when the other has a friend over, or, if you're sanity can handle it

so that they each have a friend over at the same time. My mom did this with my brother and I as we got older. Of course it doesn't always work, and in that case, I think you're doing well by not allowing your girls to shut each other out of the room and trying to encourage your daughter without a friend over to interact with you.
You mention the issue specifically with one of your daughters and her friend. Does your other daughter have friends over on her own at all? Do you think her feelings could be stronger because she feels like she doesn't have a friend as close as Justine and her friend?