Hi aykess,
I'm not in the configuration you described per se, but the MFM V I'm in (Brother-Husband, Snowbunny, and me) looks a bit like it. Snowbunny is legally-lawfully married to Brother-Husband, but thinks of me as a second husband. Meanwhile, we two men could date polyamorously if we wanted to, but so far that hasn't happened, and it seems unlikely that it ever will happen.
So Snowbunny is kind of like a poly who is married to one mono and "dating" another mono. Maybe not in theory, but in practice.
What's it like? Agreeable. Snowbunny is kind of like the person "in charge;" she keeps things organized and is conscientious about giving fair portions of her time and energy to each of her men. It's a big job and a lot of responsibility, but that doesn't seem to scare her. Meanwhile, the three of us seem to get along pretty well. We share a home.