I wanted to add that co-parenting can be done with more than just two parents... I don't know if that would be co-parenting squared or what? lol
But anyway... DP and I actually communicate a lot with his ex and her BF about different things with DSD. But we don't neccessarily do everything the same.
Violet said it best. DP, me and DSD are one family unit, and DSD's Mom, her BF and DSD are another family unit.
In both there are two parents, comprised of one bio and one step, but they work together to raise the child in their home, and the other set does the same... on drop-offs all four of us sort of compare notes on how DSD is doing and such, and if there are any big issues that we want to get more input on, we bring it up and discuss it.
In the begining I certainly did not have as much say as I do now... but over time my opinion became respected, and even DSD's Mother asks my opinion on things at times. We have always been civil to one another, which from what I know I should be grateful for. She does a lot of things that make my head want to explode, and certainly not things I'd do with my own children, but overall she is a good Mother.
But both DP and I do not bend to her rules for DSD because DSD's Mom lets her do whatever she wants and lets her eat McDonald's like every day, and lots of other things that we just don't agree with. Actually that was a big conflict in their marriage... they were never on the same parenting page.
DP and I are blessed that we think alike on a lot of things on how we want to raise the children.
So after all that... I think it is quite possible for ALL parental figures, step or bio to parent together, but not neccessarily defer or have to do everything the same, but still provide a balanced environement for the children in two functioning homes.
I can see how maintaining the view that the new relationship/marriage is only temporary (without any real reason to believe so other than statistical anecdotes that may be indicative of issues not inherent in the relationship being examined) can serve as a roadblock to realizing that healthy ideal and might create tensions that don't serve any productive purpose.