well, you will need to add a category column to the cart object to be able to trigger the discount, you will also need to have a discountAmount column. in the add to cart button, you will need to assign the discount amount column to be the same as the price, and assign the category to the category column.
the discount amount column should only be given a value for items the discount should apply to.
lets the category to apply the discount to is 3 for example sakes:
for the trigger, you will use your basic trigger for applying a discount code, based on the session value, but you will need to add a second trigger as well:
Number of Items with a specific column value-
if the total number of items where 'category' has a value of '3' is '=' a value of '1'
for the calculation, use:
based on column subtotal-
subtotal for column 'discountAmount' 'plus' '0'