If taken just literally, in Buddhism, mutual causation means that the two are mutually causative. Let's say that everything we experience in the present moment comes from an antecedent cause, and everything we do in the present moment seeds the cause for all future results. From this point of view, the cause of the present result is the future result, and the present result will also become the future cause.
For example, if a person is born with a compassionate nature, this must come from the accumulation of good causes in past lives, and this goodness is rewarded. This person, in turn, will be able to cultivate many more good causes and good karma because of this good root habit, and will be able to obtain good fruits again in the future. This can also be understood as mutual karma!
This is my personal opinion, just for your reference!