Hi,
There are several triggers in the coupon/discount system.
- onBeforeCouponLoad( & $coupon, & $do)
- onBeforeCouponCheck( &$coupon,&$total,&$zones,&$products,&$display_error, &$error_message, & $do)
- onAfterCouponCheck(&$coupon, &$total, &$zones, &$products, &$display_error, &$error_message, &$do)
In your case, I will recommend the onBeforeCouponCheck or the onAfterCouponCheck.
You have the access to the products of the cart so you can check the ids, check if there are your two products and access the discount (by not setting the variable $do to false).
The value and the basic rules would be already implemented in the coupon, but your plugin will add the product check rules.
Regards,