The word that names the relationship between two women that share a man. In other words "lesbians by proxy."
It could be the relationship between the wife of a man and his mistress (whether they have met or not.)
It could be two women that share the same man at the same time.
Or it could simply be the relationship between the girlfriend of a man and his female roommate. "My husband spends all of his free time with my proxbian and it wouldn't bother me as much if he would let me share her every once in awhile."