The right way to do polygamy is to have him provide each of you with your own house, transportation and a thriving business of your own. There has to be something worthwhile in it for you to be willing to share a man. What happens when the man dies and both of you are stuck in the same house broke sitting on that cheap sofa? At least if you have your own stuff and are self sufficient, you can take your kids, bounce and move on with your life.
Every poor choice has to be worth something. If you must date a married man, then you better come out of it with investments, a condo and car in your name. When he leaves or you get tired, you can cry in your own home and not outside in the streets with nothing but a suitcase of clothes and bags to your name.