Magento 2: how to put extra text / drop down for decimal qty

I need to customize the addtocart.phtml in order to add an extra field for decimal qty.

I want to use the actual input text for quantities in order to use ONLY unit and I need to create another drop down in order to select half quantity. (See picture)

Here is an example of what I need:
https://www.fabric.com/buy/eu-376/north-american-wildlife-abstract-ocean

As you can see, in this product page, there is qty in yard and then a drop down for 0 and 1/2 yard.

I need exactly something like that!
How can I do it?

PS I don’t need to keep track of the inventory, but of course the quantity have to calculate the right price.

Thank you!!