How can I use two different templates for a single entry?
You do not need to create a separate section for course work.

What you want to have is two different urls, that both display the same entry, but in two different ways, with two different templates. So for example /course-slug will be the url for info page for the entry with slug course-slug, and /course-slug/lesson will be the url for the work page for the entry.

If you think of this as two different routes, you have set one of those routes up in the course section. You can set up a second one as a dynamic route. You want to route [slug]/lesson (uri) to _work (template) (or whatever your template is). Then in the _work.twig template, you will need to fetch the entry from the slug:

{% set entry = craft.entries.section('course').slug(slug).first() %}


Check if no entry is set, and if not {% exit 404 %}. And then proceed as normal.