I don't quite follow your description of what you want to do, but it may help to explain how discounts work.
At any one time, a buyer's cart can have at most one discount applied to each of the following, in any combination:
- The cart's Item Total (before tax/shipping);
- The cart's Shipping amount;
- The price of each item in their cart.
Discounts configured for All Cart Items can take a single flat amount (or a percentage) off the order total, but these cannot be limited to orders for any particular product and cannot be combine more than one such discount in the buyer's cart at once.
Product-specific discounts modify the per-unit price of a particular item in the buyer's cart, but they cannot take a fixed, flat amount off the order total regardless of item quantity; however, you could disable "Let buyer edit quantity" in the product's settings, so it would not be possible to order more than one unit at a time.
Other details about discounts are covered on this help page:
If you have a buyer with multiple coupon codes they want to redeem, they could just place separate orders (once for each discount), or if they email you with the coupon codes they want to use, you can delete those and create a new, single coupon code with their combined value for that buyer to use.