@Doug9 

Since the parameter doesn't have square brackets at the end of the name, it typically means that it only accepts one value.  Thus, when using that endpoint you would need to make two API calls (one for each of the workflow states you want) or make a single call without using the worfklow_state parameter and then filter the results to only the ones you want.

Also, is there a reason you aren't including "pending_review" as a workflow_state to look at?  This workflow state is used to indicate quizzes that have been submitted but still have questions that need to be graded by the instructor.