The course_id is part of the path in the URL. As such, it cannot accept more than one value at a time.
Properties that can accept multiple values have [ ] in them. That's also used to indicate properties of an object, in which case you're passing multiple properties rather than multiple values. The properties that take multiple values typically have empty brackets at the end [ ]. For example include, student_ids, or assignment_ids.
The Live API was designed for exploration, testing, and understanding. If you're going to use the API in production and do something for a lot of courses, you should probably write a program to make those calls.