I don't know much about the .450 Alaskan but the .50 Alaskan has been used successfully for head shots on elephant. These however are essentially lever gun cartridges for shorter actions and you may still be limiting yourself in a double rifle.

If your gunsmith has already fitted .458 caliber barrels to your DR, why not ask him to chamber and regulate a .450 No.2 NE for you? This lower pressure cartridge should work fine in an action built strong enough for the .450 AK and you can use it to stop anything on Earth. A good old .45-120 with custom smokeless loads to throw 480 gr. bullets at 2100 plus FPS would also do the J.O.B.

Then your 9.3 could be your plains game and backup DG cartridge. Problem solved.