WooCommerce: multiple input field for multiple product variations

Just an FYI to other people who found this: I was able to achieve this using Grouped products instead of Variants. Simply add the main product (the Wine, in this case) as a ‘Grouped Product’. Then, add the 2 case sizes as simple product, and in the ‘Advanced’ tab, choose the Grouped product from the ‘Grouping’ dropdown.

I ended up with a page that looks like this.

Hope this is helpful to somebody.