...if i am not mistaken, california rigbys have done exactly what you are proposing, but charging 30k+ for shotgun-based boxlock double rifles...

...my understanding is that rigbys' double rifle formula consists of a merkel boxlock shotgun receiver, a pair of pac-nor barrels, and the firm's own wood stocks...

...i can't see why you could not do the same...tho regulation of the barrels may pose some challenges...