NitroX: the single safari rifle is something that I too am smitten with... When I ordered my bespoke Heym double, it was a .375 Flanged Magnum with a second barrel set in 450 3/4" Nitro Epress. I got a sturdy Pelican model 1700 high density foam shipping case and have cut it out so it holds the rifle and both barrel sets in a broken down state. The total weight, complete with two padlocks is 32 pounds.

As long as it's ok to bring ammo to the destination country in two different calibers for the one rifle, it would seem a great way to travel. The worry of having the wrong barrel on the rifle at the wrong time based on the game that one runs into is not too much of an issue as I see it. If hunting DG, I'd have the .450NE barrel set mounted on the action exclusively and I wouldn't be inclined to shoot any incidental game because I would be solely and strictly focused on Buffalo. Once the buffalo is in the salt, then I'd switch out to the .375 FL Mag barrels and focus on plains game. During North American hunts, I have the .375 FL Mag that gives me a bit more than point blank range, and is a caliber a bit more suited to North American game and assures that my double sees some good field use here at home.