hi, you are facing the task to edit one tables data as well as an associated relationship table by id on a single apex page? you think that a multiselect list control will do for the relationship table entries but do not know how to integrate it? iff so, you are at the right stop. the following article will give you a screenshot tour of a step-by-step procedure with appropriate annotations.
placing and feeding the multiselect list control
#1 at first, on the dedicated edit page, create a new item within the form region.