Not an unheard of idea? But, probably easier to have rifle barrels installed into a double shot guns breech assembly? I read an article, years back, about a gunsmith who was installing rifle barrels into shot guns breech assemblies and called it mono-blocking? The problem with this and the expense starts mounting when it comes to accuracy? Regulating the barrels(getting them to shoot the same place), is what dooms most cheap manufacturing.
As a side note, I've always dreampt of owning a Brittish double rifle in one of the African calibers as I'm sure anyone who grew up reading Rourk or any PHs, or African hunting, books would.
I always thought a double 30.06 would be the ultimate deep woods deer rifle?
But, maybe manufacturing techniques have progressed enough to get good accuracy and cut costs?