The AllProWebTools ecommerce system allows for 2 different types of coupons, "exclusive" and "combinative".
Exclusive coupons cannot be combined with any other discount, this includes other coupons and quantity discounts. When a customer applies a coupon that has been configured as "Exclusive" - all other previous discounts are removed from the order and only the exclusive coupon is applied. Exclusive coupons can only be applied to one qualifying item per order, so if the coupon would normally work with either of 2 items in an order, it will only apply to one of the items.
Combiniative coupons CAN be used with other discounts, this includes other combinative coupons (not exclusive coupons) and quantity discounts. Due to the combinative nature of these coupons - they can also be applied to multiple items ordered all at the same time.
I understand that this may not provide you with a perfect solution for your needs, but these are the limitations of the AllProWebTools ecommerce system.
If your coupon is currently configured as "exclusive" we can change it to be combinative if you would like - though this would likely introduce other undesirable behaviors. For instance, a customer could potentially use the coupon and another coupon together to derive a larger combined discount than you originally intended.