Take a typical codependency problem. Person A depends on person B. Person B enables person A.
It seems to me that most people would say that each is half responsible for the situation but fault arithmetic doesn't work that way. In fact, person A is 100% responsible for the situation and person B is also 100% responsible. Here's why:
If person A decides to stop being dependent, the situation ends. So person B's decision is not sufficient to the problem. If person B decides to stop enabling, the situation ends. So person A's decision is not sufficient to the problem either. In fact, since either person can end the cycle with their decision alone, both of them are necessary.
If your participation in an activity is necessary for its continuation, you carry full responsibility for its consequences. So division of blame doesn't work like division of, say, a bushel of apples.
(9.31 gallons) / (2 bags) = 4.65 gallons per bag
...but...
(100% blame) / (2 necessary participants) = 100% blame per necessary participant
