The practice of quasi polygamous 'parallel families' is still alive and well in the Dominican Republic, but I get the impression that it is not as widespread as it used to be.
We had a great uncle of my husbands staying with us some time ago, and he spoke about his parents' set up. He is in his 70s and was the youngest of a huge family, so we are talking early 20th century here. His father had two 'wives' - whether he was legally married to either nobody knows, or cares - and they lived next door to each other and the entire set-up was completely open. The two women got on OK, apparently.
I know several men of my generation (30s and 40s) who have children by other women, some openly, some covertly. The case I know best is a friend who has not divorced his original wife, and still spends some time with her, but has a younger family with a younger woman. This second family is his main emotional focus but he is still committed financially to his first wife and daughter. Both women accept the arrangement and look after each other's kids when necessary, but don't have anything to do with each other. In effect, this man has a first wife and a second wife. The only difference is he is not divorced from the first one and still maintains some sort of relationship with her.
It's not really about legality, it's more about what society here - and the women concerned - are prepared to tolerate.
Chiri