Two grand does seem to be a very good deal in that rifle.
There are quite a few makers who will knock out a 12b double rifle. I was handling a very very sweet Verney-Carron 12b SxS at Reno SCI.

LGF, if there was enough wall to the tubes on your 12b shotgun, I'm sure it could be done, but I doubt it's heavy enough in the walls to make it.
Gun barrels tend to be pretty thin at the center section, and then *if it could make it there and past proof* it would surely need radical re-regulation and a big facelift on the sights.
Best to go whole-hog.

Cheers
Tinker