Assuming I understand your question correctly and the coupon is a separate discount and is not related to the combo discount mentioned in your original post.
You want the coupon to be applied to only one of two desserts (house and premium) in a sequence of either the least expensive item or more expensive item when both are ordered on the same check. There are two things you want to consider:
1. Do customers need to purchase an item and a dessert to use the discount? Or coupon can be used on one dessert item without an additional purchase.
2. Can multiple coupons be used on the same check (3 coupons for 3 desserts on the same check)? Or limited to one coupon per check (3 coupons available and 3 desserts ordered but only 1 coupon are allowed).
Here is an example for a discount when:
a) additional purchase is required,
b) only one coupon is allowed per check,
c) and deal with sequencing.
You probably want to set it up as:
1. BOGO discount type
2. with a purchase of any item or a specific item to use the discount
3. Choose how you want the discount to be applied: first item eligible as input, least expensive, or more expensive.
4. Make sure to set "Eligible of # Get Item" to 1.
If more than one coupon is allowed per check or additional purchase is not required, simply set up a fixed $ off discount with house and premium dessert as the only eligible items and manually apply discount. This won't deal with sequencing so you will need to train your employees to always apply discount to least expensive items first or more expensive items first.
