A woman married to a man, who then marries one or more women. Or a woman married to a woman, who then marries one or more men.
Bob is married to Jennifer, who then gets married to Jessica. A basic BJ-J.
Jennifer's relationship is an example of polybiandry.
e.g. Valarie is married to Jade, who in turn gets married to Brian. Jade is engaged in a polybiandrous relationship, with or without the knowledge of Valarie and/or Brian.