I have not found a way to show what your asking in a Descendant Report or Chart.
My experience is that FTM shows the child AND ALL DESCENDANTS OF THAT CHILD for EACH PARENT of that child in a DESCENDING REPORT. This is regardless of the choice of type of relationship (ie natural, adopted, step, foster, etc).
If the parents are both descendants of a common ancestor upstream (whether by cousin marriage or adopting "within the family"), the children - and all descendants of that child - will be listed twice (or even more times if multiple cousin marriages).
When you run an ancestral report in FTM, FTM will include ANCESTORS based on the Preferred Parents choice.
My experience is that you must go to File > Person > Attach/Detach > Detach and detach the child from their adoptive parents. You can re-attach after running the report if you wish.
This is why I never ATTACH step-children as children of step-parents. I record the children to their natural parents, if known, but not foster or step-parents.
This is another one of my annual enhancement requests to FTM that seem to go unheeded from year to year.