The Discount system is only able to apply one discount per order.
For now, I would suggest updating the items price to apply the discount that way instead of using the coupon system. You can add copy to the items description to describe Include the Original Price and Sale price.
After the holiday season, when Cafe Commerce traffic is a little slower, we will look into improving the coupon system to allow for multiple discounts to apply to an order. It would be a rather large task to take on now with the current traffic, and would risk breaking other aspects.
