9.3 x 62 and 270 Winchester is a combination that is reasonably often used in two barrel sets for Mausers and other rifles. Hartman and Weiss have made this combination before and still do now. The cases are so similar that they will both feed without any problems and as they are the same length and bolt face as the 30-06, they will fit nicely in the magazine and can use the same follower with maybe a small amount of modification.

What method are you going to use to lock the barrels and the fore-end ?

Waidmannsheil.