What is the relation between the two wives of a man called?
According to this book, they are considered "co-wives."
Amongst polygamist American sects, apparently the term sister-wife is used when several women are married to the same man. These women are not actually sisters by blood, but more like sisters-in-law, and they refer to each other as sister-wives.
There’s even a “reality TV” program about one such plural family, named Sister Wives. From Wikipedia:
Sister Wives is an American reality television series broadcast on TLC that started in 2010. The show documents the life of a polygamist family, which includes patriarch Kody Brown, his four wives and their 17 children. The family began the series living in Lehi, Utah, but has since moved to Las Vegas, Nevada.
In the Old Testament, they are called rival wives (Leviticus 18:18, NIV). KJV uses a descriptive phrase "to vex her."
Neither shalt thou take a wife to her sister, to vex her... beside the other in her life time.
There is a description of the marriage between a man and two sisters: a sororate marriage. This word is close, but perhaps not exactly what you are looking for. If you called the relationship "sororate," you would probably be understood.
Fellow-wives or rival wives can also be used.