Unfortunately the linking works the other way, multiple jobs to one customer.
Is the end result to be able to view which customers have purchased access to the training course?
You could use different 'Items' for the different courses, add the items to the invoices, and customise reports to view which customers have purchased the various courses.
Alternatively, you could set up 'Customer Types', and assign each customer to a type, although you can only have one type set per customer. Ie, if you had 'Course 1' as a customer type, and 'Course 2' as a customer type, 'Customer ABC' could not be linked to both, unless you also had a type 'Course 1+2'.
Or if it the end result where more cost focused, you could set up a different 'Class' for each course and then assign the costs and invoices to the different classes.
There are a few features you could use depending on what it is you actually want.