Because both people are maintaining the pattern, no one is to blame. Every player in a melodrama is equally responsible, and none is at fault. Systems theory is how counselors keep their balance, and how members of couples and families keep their balance. It provides a compass in the stormy waters.
Another way to put it is that it doesn't matter who started it, or who's at fault. What matters is what each person is doing now to maintain the pattern, and what small thing each would be willing to do different. Another way to put it is that it helps to have a functional rather than blaming point of view.
If a person is a member of two systems, the same dynamics will tend to operate in both systems. A supervisor can get a taste of the flavor of the relationship between the supervisees and their clients. Another example is that a man will tend to have the same kind of relationship with his wife as with his mother, and part of it's because he's there in both relationships.
Changing part of the pattern, or intervening in a pattern, isn't predictable in a linear way. What is predictable is that the pattern as a whole will change, that it will exhibit behaviors like homeostasis and organic spontaneity, and that positive and good-faith intervention will generally have positive results. Systems react to stress like any organism, and the same things help any organism in stress.
To affect a pattern, it's technically sufficient to change any part of it or to intervene anywhere. But this isn't to say that it makes no difference what you change. Changing a relationship is a delicate art. And intervening is an artform of such subtlety that one can keep getting better at it for a lifetime. (Milton Erickson was, of course, the master of intervention, and one can hardly do better than study his work and that of his successors.)
One can affect a system by working with only one member of it. A wife who comes alone to therapy is giving the counselor plenty of handle to perturb the husband-wife system, whether the husband wants it to be perturbed or not.
A change in one system will tend to echo into other interlinked systems. A supervisor who accepts a counselor without judgement makes it easier for the counselor to accept the clients that way. A person who talks freely and openly with their counselor is doing self-modeling for better communication with their friends and loved ones.
Systems theory also applies directly to individuals. A set of beliefs and the behaviors that maintain them form a perturbable system. The entire theory of counseling individuals is contained in that sentence. Questions, feedback, insight, problem-solving, reframing, mirroring, binding, humor... they all aim to disturb systems in healthy and positive ways.
Here's a version of Systems Information you can download: