The wiki states that
One coupon per person per journey leg is required - so, for example, TXL-ATH-SKG and return via the same route would require four coupons per person.
I cannot confirm this; I were booked DUS-SKG-ATH vv (not on the direct flights but with a 2hr overlay in SKG) and they only invalidated two of my four vouchers, although I would have expected that all four vouchers were needed to book such an upgrade.
Did they just make a mistake or can anyone confirm?