Browning doesn't make extra barrels for O/U's. If you value your Citori call Browning tell them what you need and are trying to do and they will advise you one way or the other. But the cost will be somewhat high but you're safer with the manufacturer being involved. They may also direct you to one of their service centers to speak with a gunsmith.
i figured they offered one since you can get one in a combo package.i could have swore i seen one for sale at one time or another. i may have to buy another o/u barrel and cut off the top barrel and machine a rib for it.