The relationship of two wives sharing one husband is called polygyny. Polygyny is a form of polygamy, which is a marriage arrangement where one person (in this case, the husband) is married to multiple spouses simultaneously. It is essential to note that polygyny refers specifically to the scenario where a man has multiple wives, whereas having multiple husbands is known as polyandry. Both polygyny and polyandry are relatively rare compared to monogamous marriages, which involve one husband and one wife.