JPK
Since you are a "Pioneer" in using a rimless double, you are a natural for a 375 H&H double. I say go for it.
I recommend a Chapuis 375 H&H, scoped of course.

However should you have a sudden conservative streak, I can say that a Chapuis 9,3x74R is a BAD TO THE BONE hunting double rifle.

But with a 458 double and a 375 H&H double....
Ballistically speaking you have it covered, and you can find ammo any where ammo can be found.

Baised on what you have said about your Marcel Thys 458 functioning 100% I would not even consider changing it to the 450 Nitro.