Yes,  even if only one parent goes through the course, the results will be evident.

What tends to happen is when one parent changes the way they deal with various issues and uses his or her new skills, the other parent can see improvement, and they tend to copy the techniques. Also as the other parent sees the results, they may be more inclined to participate on the course too.

Obviously, if both parents do the course together, they will be working from the same page on the various training issues. This offers the child a level of consistency and congruency, which prevents playing one parent against the other; they learn to appreciate that both parents say what they mean and mean what they say!