Successful completion of any cognitive task requires selecting a particular action and the object the action is applied to. Oberauer (2009) suggested a working memory (WM) model comprising a declarative and a procedural part with analogous structures. One important assumption of this model is that both parts work independently of each other, and previous work has indeed reported evidence for the independent selection of memory lists and tasks, for example. The present study focused on the selection of single items in declarative and single responses in procedural WM. To this end, WM updating tasks were implemented as Task 2 in a Psychological Refractory Period setup in 3 experiments. The prediction based on the locus of slack logic is that item switch costs are present with a long but not with a short stimulus onset asynchrony. Contrary to this prediction, item switch costs were neither absent nor at least smaller at the short stimulus onset asynchrony. These results help to further constrain the WM model and thus to refine its computational implementation.