Hi Jacque
Activity restrictions allow topics or activities to be restricted to members of an audience and also by date ranges and allows restrictions sets to be built
You could user several courses and enrol users in an audience with cohort 1 users and restrict access to the content and have another topic in the same course restricted to cohort 2 and another date.
The course completion could be based on meeting a course grade if they are graded activities or marking yourself complete if not.
Regards