There was a good article with photos etc in the "Double Gun Journal" a few years ago on just such a project. The author built a 22RF double rifle on a boxlock 410 side-by-side. He had barrel sleeves made up with the 22 chambers excentric(sp?) to the 410 originals so the centrefire pins would hit the rim of the rimfire chamber. He had some interesting hurdles throughout the project (particlarly in regulating the barrels), but was happy with the result. I no longer subscribe to that rag, but will try & find which edition the article was in.

I would suspect that to have barrels specifically made up would be extremely expensive. I'm no gunsmith, but I imagine it would also be prohibitively expensive to have have chambers concentric to the outside barrel profiles, as the firing pin geometry would have to be dramatically modified.

So yes, it can & has been done. Think of the possibilities with 4 rimfire cartridges now available?!