I have the same problem as the OP. I have seven separate courses who share some assignments, but not all. I have two courses that are the exact same course, meeting at the same time, but the PowerSchool codes are different, so it creates two courses in Canvas. For the second case, copying to sandbox, then commons, then the other course is actually more work.
I haven't tried this solution yet, because I haven't received my courses for the fall yet. However, even if it works, this solution is still incredibly tedious. Can Canvas please add an option on an assignment to assign to multiple courses? Adding files to multiple courses at once would also be such a huge time saver. I am pretty sure it can be done, because it is a feature in Schoology.
Thanks for considering!