Discount Coupons are based on the current Price of a product that is then offset by either:
1 Amount Off
2 Percentage Off
3 Free Shipping
There is nothing built in, at this time, to skip existing Specials, Sales etc.
I can understand not wanting 2 Discount Coupons used at the same time, but I don't understand not allowing a Discount Coupon to work on the current Price of a Product ... that seems a bit odd and not something that will appeal to customers ...
To make something like this would require quite a bit of customization of the code in addition to detailed explination to the customer as to why that Product that was $100 and on Special for $75.00 when using a 10% Discount Coupon loses the Special Price and results in a $90.00 price ...
What was the point of the Discount Coupon that got me to come into the store? :unsure:
This could be designed ... I don't know how well that is going to go over with the average customer ... and the explination on how this works will probably have them leaving rather than buying ...
Perhaps a post in the Commercial Help Wanted to assist you in getting this custom code written for your shop ... although I would recommend having a think on this for a bit ...