Magento 2 Coupon Code – How to Validate Coupon Codes

You can do one thing when any user enter the coupon code for particular product and press apply button you can put ajax link url will be

http://localhost/magento/index.php/checkout/cart/updatePost/

and create one observer checkout_cart_update_items_before write your code for apply coupon.

As per as magento system,you cannot apply multiple coupon on a cart.

If you want then need to use

3rd partly extension like amasty

Or

you can use this answer:

Magento Multiple coupon apply on cart