Some can be fitted and some cannot, without very expensive alterations. IMHO, there are very few double gun smiths that can really fit a set right.
Just because the barrels close tightly and there is no noticeable play with the forend off doesn't mean they are properly fitted. There should be a very high percentage of contact between the barrel's breech ends and the standing breech. I once paid a nationally touted double gun smith to fit an extra set. He bragged on how easy it was. When I received it I could see daylight all the way to the bottom on one barrel and the other barrel had less than 30% contact with the breech. Totally botched.
I have attempted to fit a set myself and failed in doing it properly. So, I somewhat understand the geometry and mechanics involved. There is one American smith that I would trust to do it. But, he is so self-centered and egotistical that I have "sworn" to never deal with him again. Put simply, it's not for the faint of heart, whether you're doing the work or having it done.
