To create a return with multiple products, don't use the "Quick Return" form.
Instead, create the Customer first by clicking the "+" on top of the page and then "New Customer".
After the customer is created, click the "+ Add" button on top of the customer page and select "New Return".
After the first product is added to the return, an "Add" button will appear in the Products section.
Click the "Add" button to add more products to the return.