Hi and welcome!
You might try searching "quads" as well. It seems to me like some couples on here have managed to build a non-hierarchical model with other couples and become a quad. I personally believe it is possible, and want that for my life as well.
I suspect that it could take time for all parties to build trust in each other, and commitment that no one is being left behind. But I think that's true of any relationship. From being in a triad that went badly, my advice is to go slow, respecting everyone's emotions through spending time with one another, allowing all parties to have "alone time" with each other. Also, the pre-existing problems of the relationships come into whatever is being built, although (and this is something I've written less about) there are both opportunities for poly to help HEAL these problems as well as they become more clear and obvious because of the insecurities and worries poly can bring up.
In my dreams of what I would like, I can see that being with another couple is ideal, as all parties can slowly build a relationship with one another. I imagine that as a couple, myself and my partner can get to know another couple slowly, and then through conversations with each other and with the other couple, move things to a non-hierarchical level when I feel the trust is deep enough.
Where are you at in your poly journey? Have you known this couple for a long time and been intimate, or are you long time friends and just starting to consider poly with them?