When you enter a code on the front end, the system will look in the database for a coupon with that code. If one is found, it will check each restriction one by one:
Is the coupon quota ok ?
Is the coupon end date in the future (or not set) ?
Is the coupon start date in the past (or not set) ?
Is the coupon product in the cart (or no product restriction in the coupon) ?
Is the coupon category a parent category of one of the products in the cart (or no category restriction in the coupon) ?
Is the current zone (the state/country of the address of the user) in the coupon zone (or no zone restriction in the coupon) ?
If one of these questions' answer is no, then the code is rejected and an error message is displayed.
That's how the current coupon system is built. As you can see, even if two coupons with the same code in the database, only the first one will be taken into account and the second coupon information won't be used. Thus, it is not possible to have two coupons with the same code for 2 different products with different values.
You could only accomplish that by either:
Using one coupon restricting to a common category for both products. But you will have the same value for the coupon
Using two different coupon with different code, restriction and value