That would actually take a bit of work, and would require source code and a dev familiar with MVC.
To start with, the contents of the order notes view would have to be moved into the checkout index view, and the form tag on the checkout page would have to be moved to also include that new content. Then the properties from the OrderNotes view model would have to be moved into the CheckoutIndex view models, along with the logic for populating and saving them. There'd be a little cleanup after that, but that would cover most of it.
We get asked about this fairly frequently so I've put in a request to look into changing that behavior OOB some time in the future.