British Airways offers '2 for 1' mileage redemptions (one per year) to holders of its Amex card who spend £15,000.

Most of us on the BA board use these to get 2 long-haul First or Business flights for the miles of one (up to £7,000 / $15,000 value per ticket). The BA staff who post on the board will tell you that most people use them to claim a second free ticket to Paris or Nice (retail value £69 / $110).