Depends on the competition in your space. If you are competing with very comprehensive courses with lots of hours in them, you might have to combine those modules in order to have a comparable offering.
If you don't have competition, then you could consider splitting them up. It's not usually a good idea to have courses that depend on the student having taken other courses first, though. You'll end up with a lot of unhappy students who didn't realize that when they bought it. A lot of people buy Udemy courses on impulse, so it's best to ensure your courses can all stand on their own.