I have been looking here for an unsingle barrel for my XT for over a year. They show up occasionally and are usually snapped up instantly, and they are usually pretty pricey too. Last one I saw went for $2000!
I'd say, do what I do. Enjoy the gun the way it is. And forget about the other barrel.
Also the unsingle barrels all have high ribs so unless your gun has a high rib already, you end up with 2 different height ribs which is harder to fix.
The only other viable solution is to buy an XT combo which show up here more frequently, and are usually priced around $3500. And then sell your's