Of course there are no reasons not to built a double rifle on a shotgun action but:
A REAL maker takes an action 'off the forge'; leaves the walls and the lumps extra thick; puts in bushed strikers in case of a back-fire; regulates the barrels just so;puts many hours (sometimes hundreds) in making that rifle just right (not counting the many YEARS he spent becoming a master gunmaker; always under the supervision of an elder).
Gunmaking is an art, leave it to the artists.
I love working on guns, but I never go furter than general stockwork...
Greetings,
Livinus