I understand what you're saying, but still don't have a solution. I've attached a few pics that show how I've created these flat rate discounts. Using the column values of "taxable" for a taxable item and a column value of "coupon1" for items that are part of this special. I think you can get a sense of the perhaps mickey mouse way I did this. Since the prices of all the products (coupon1=1) on special are the same, I was able to make it work with the flat rate option. Trying to take a discount of 50% off any item with a coupon1=1 only when a second coupon1=1 item was added to the cart was not working. The problem I had was only the second, third, and so on items with a value of coupon1=1 should have the 50% discount, not the first.
if productA (coupon1=1) only - no discount
if productA (coupon1=1) and productB (coupon1=1) - discount on productB only
if productA (coupon1=1) and productB (coupon1=1) and productC (coupon1=1) - discount on B and C only.