It does make sense.
So here is my story (very short version-but you can find more by looking through my first posts).
I am married (10 years).
I have a boyfriend who also lives with us. I've been friends with him for nearly 17 years.
I have a daughter (18 years) and my husband has a son (13 years) who we raise together, whose other parents are primarily uninvolved in the children's lives.
My husband and I also have a son (9 years) and we are raising our daughter (2 1/2 years). The daughter is biologically the child of myself and boyfriend. BUT we had her by way of artificial insemination prior to him being my boyfriend.
ALL of the kids, extended family etc know the details of biology for our children, but ALL the children know Maca and I as mom and dad. They all call my bf by his given name.
by some accounts bf would be a secondary.
We've lived together as a family unit for years, my sister who also lives with us as well.
We function as a family of 4 adults, but there are MANY details that are primarily shared by Maca and I (ownership of property/parenting legal rights for all children etc).
So in many ways my sister and BF are secondary, and yet ESPECIALLY for the kids sake-they function as additional primaries when it comes to raising kids (making rules, upholding rules, helping with rides, homework, scheduling etc). Both are listed as emergency contacts for ALL the kids on ALL paperwork and all four of us are capable of caring for any and all of the kids at any time and the kids are confident and comfortable with that...
So I guess-from that point, maybe a more specific question would give me an idea of what you are looking for informationally??