The reality is that there were two people who committed the serious mortal sin in which they both rejected God’s original friendship relationship aka the State of Original Holiness aka State of Sanctifying Grace. And both two people suffered the results.
Even if the second person Eve committed the first mortal sin, she would be guilty of that sin, but not guilty of Original Sin because she is not the original person on planet earth.
It is hard to grasp the concept of Original when there is original this and original that. Perhaps, when we consider the role of the first human who was the first human in the first friendship relationship with his Creator, we can understand why Original Sin belongs to him. Genesis 2:15; Genesis 2:18; Genesis 3:9-11
It does take two people to produce descendants which will have their parents’ wounded nature.