I have been able to use both a discount coupon and a voucher on tickets with CO. However, I must do the ticketing directly with a ticket office or over the phone (new e-certs) as the travel agents' CRS can't handle the double discount.

My company also works with Carlson Wagonlit and has an agreement with Continental. I am able to take advantage of the discount fares and use a voucher, but not a coupon.