I tried to do the exact same thing - turn a Citori into an-unsingle combo. It was impossible to find an unsingle barrel. Moreover, several people mentioned that there was a high probability that any new barrel I found wouldn't fit the old receiver.
So I bought a compete Citori unsingle combo, and whadaya know - the new unsingle barrel would NOT fit the old receiver. So I sold the old Citori at a reasonable price and now I have the combo I wanted.
I think, in the last analysis, what I did is the only way to get it dome. It's for sure that Browning won't sell an unsingle barrel separately.
BTW, the rib adjustment mechanism on the unsingle barrel is like no other I've seen. Also, the rib on the double barrel is not adjustable.
Ted K.