What was the first double rifle nitro express caliber in history?

Not the first double rifle (they have been around since muzzle loading days) but the first "Nitro Express" was created by Rigby in 1898 as the .450 Nitro Express. This used the newly created cordite propellant in lieu of black powder (that is the nitro part of the name). There was soon after several variants and other companies created their own versions.