This is purely anecdotal here and taking me and my friend as something of a case study.
Myself and my best friend have known each other since we were three, grew up together etc. Our parents also divorced within a year of each other.
Both sets of parents co-parented.
My friends mental health has been in tatters ever since. Mine took a pretty big bump but is now fine (in fact I'm probably the most content I've ever been, and a draconian secondary school was the major trigger for my mh problems).
We are both 28 now and were around 10/11 at the time of the divorces.
The difference between the two sets of parents, from my perspective and with hindsight is this:
My parents - there was no cheating or step parents involved that I was aware of as a child. My parents both genuinely get along, are great friends and think very similarly. The narrative I was given was that they still like each other, perhaps even loved each other but just weren't compatible living together. So there was really no need to choose between them. It was as though they were like friends in school who couldn't sit next to each other in class because it just didn't work, but still played at playtime.
I never heard any shouting or arguments. The home I grew up was never sold and I didn't have to move as such. My mum bought a new house near a better secondary school, I commuted to primary school in year 6 (my mum drove me there) so no upheaval there.
My dad kept the first house and all my stuff etc, I spent every Friday Saturday and Sunday night there. Weekdays at my mum's.
This whole setup wasn't a big change either really due to their working patterns. My mum worked weekend night shifts as a nurse and my dad worked away(other side of the pennines) mon-friday.
So between my parents being civil, not sniping at each other, weaponising me etc, the upheaval for me as a child was minimal.
I later found out that it was a combination of my father's weed habit and my mum cheating with a colleague that kicked it all off. But that was much later, and incrementally so it was much easier to get my head around. I was shielded from that as a child and I think that's quite an important factor.
They were also quite happy to take me on days out etc together and managed to get along perfectly well (in fact they have got along better since they divorced. They regularly go to concerts together now and are good friends)
My friend on the other hand -
There was cheating involved that her and her siblings were aware of from the start. The narrative of their father abandoning them and their mother for a woman who openly disliked them was peddled to them. Their step mother also treated far worse than her own children. Even I as a relative outsider thought she was unpleasant.
They also heard the arguments before the split. Lost their childhood home etc.
Her parents did 'co-parent' but mostly for the sake of the children, and the toxic atmosphere whenever they were both in the same room was palpable. They clearly didn't want to be anywhere near each other, which I think is where feelings of split loyalty came from.
Her parents openly despise each other and regularly put down the other.
The three siblings were effectively simultaneously rejected by their father and became a crutch for their heartbroken mother.
The patterns of seeing the parents changed quite dramatically too as their mum had been a sahm and their father a retired army officer, so they saw much less of them. There was a lot more upheaval for them, and they were less shielded from the reality of why their parents split.
So despite both sets of parents 'co-parenting' and sharing custody without a court battle on either side, the behaviour of the two sets were markedly different, and thus the mh of me and my friend.