Is there a way to be able to add one size to the cart and then add a different size of the same shirt?

I am trying to build a form for customers to use to order products for my online boutique. I understand how to add the different sizing options and prices, but what I need is to be able to select more than one size for each outfit. For example, if a customer wants a certain shirt in two different sizes, is there a way for them to be able to add one size to the cart and then add a different size of the same shirt?

Unfortunately, that is not supported by our payment integration. The customer would need to fill the form to pick a new size for the same shirt.

So, the only option would be listing all sizes of the same shirt as a separate product each, and adding a quantity for each product.