Doubles in 405 Winchester are not unheard of. Jack O'Connor in the Shot Gun Book says that Winchester made two or three Model 21 double rifles in 405 Winchester in the early 1930s. I think that I remember seeing a double rifle of some kind in 405 Winchester in a gun store in Bruges, Belgium around 1990.

It sounds like an interesting idea. A report on how it shoots would be worthwhile.