Two rifles, but to do what?
I think that this is the right question, and there are a number of different responses...
A purely mechanical backup?
I guess that to follow the trend of this thread, if 2 is 1 and 1 is 0, then in order to truly have 2 you would need 3, right?
Yes, Murphy invites himself to your safari, but I am on
Red Leg's band wagon on this one, the odds are in your favor
I certainly respect
rookhawk's experience, whom I trust, but it still seems extraordinary. I just cannot fathom how a twig small enough to wedge itself behind the safety could sheer off a roll pin

And in this case, the rifle would still go bang after the chamber being loaded just before the final stalk...
In any case, if mechanical backup is the point, both calibers must be full fledged DG calibers. Yeah, 9.3 will do on Buffalo, but it is becoming really marginal on Elephant
I personally do not bring a purely mechanical backup on an African modern hunt. I may have thought differently 100 years ago for a 6 months true "safari" (journey) in the Lado Enclave...
A PG rifle on a DG hunt?
If, as
Red Leg pointed out, you are not on a 3 month full-bag safari, then the chances to have serious PG hunting time are slim.
Yes some DG hunts end the first day, but this is the exception rather than the rule.
Also, the reality is that most good DG blocks (especially elephant blocks) are NOT good PG grounds, not to mention that, as already mentioned, communal land is generally very, very poor PG land due to intense poaching.
As to encounters of good fortune, it seems everyone on AH needs to stop by
Green Chile's house before going to Africa to rub off some of his luck, and any scoped DG rifle in modern DG chambering will be up to the shot out to 200 yards: .416, .404, .458 Lott, etc. The lone exception may be the .458 Win in factory loads that tends to suffer from a rainbow trajectory. And of course the .375 is king, although one could argue that its true polyvalence comes at the cost of a bit of ooompth on the heavy stuff.
This being said, I do not think that there is anything wrong to have a second rifle in the Pelican 1750, because it costs nothing and does not materially change the hassle of schlepping the dang thing through airports, but do not expect to use it much, in most cases.
A different rifle for a different purpose?
Now, THAT makes a lot of sense to me, and this is what I did on my own elephant hunt on those same Hwange district communal grounds, and I was glad I did.
I had a .470 double for close hunting in dense jesse, which we did plenty of, and I carried this rifle all day, all 10 days; and I had a scoped .375 H&H loaded with 350 gr solids (I did not own the .458 Lott barrel at the time), on the shoulder of one of the trackers, in case I needed to reach out past 50 to 75 yards to take a dream 60 pounders at the other end of a clearing, or to take a representative trophy also on the other end of a clearing on the last day or so of the hunt, which is exactly what ended up happening.
Also, somewhere in my mind, the .375 H&H would have been just fine had we come across the next world-record three-toed-Unicorn (or Eland).
In conclusion...
In the vast, very vast, majority of cases, the DG rifle will not break, so a purely mechanical backup will likely never come off the rack. Yeah, the fecal matter occasionally flies into the propeller, but as
Red Leg says, it you accept the odds on a Elk hunt, there is not much reason why the odds would be so different on an elephant hunt. So, one rifle is just fine, and a purely mechanical backup rifle will likely never come off the rack.
As to the chances to bag PG worth disturbing the elephant hunting block with a shot, they are very remote, especially on communal lands. So, one rifle is just fine, and a PG rifle will likely never come off the rack. Of course, if you are
Green Chile, you will bump into something exceptional, then you will do just fine with almost any scoped bolt action .375 and up out to 200 yards. Heck, even the .458 Win with holdover on the backbone can reach...
To me,
if the first rifle is an iron sighted double in .470 or .500, the useful second rifle in the Pelican 1750, is a scoped .375 with 350 gr solids. The logic still exists, I guess, with a .404 and a 9.3x62 but I see a lot more overlap between two scoped bolt actions in .404 and 9.3, than I see between a double .470 and a scoped .375.
As to me, on dedicated DG hunts, because I seemed to have developed the habit to carry every day, all day, the double, but to systematically in the end shoot with the scoped R8, I sold the double and now only take the R8 with its scoped .458 Lott barrel, which can do anything I need from 5 yards (including stopping duties if needed) to 200+ yards. I am not worried about the odds of it breaking down on me, and if I need iron sights for a follow up in dense jesse, it takes all of 10 seconds to take the scope off.
But because I like to amortize the costs and duration of the flights to and from Africa, I always combine a DG hunt with a PG hunt, often with different outfitters and in different places, so the Pelican 1700 always carries 3 barrels and 3 scopes, picked from .458 Lott, .375 H&H, .300 Wby, and .257 Wby, depending on the trip. Heck! this is why the R8 exists, and as stated before, whether there are 1, 2, or 3 barrels in the <62" and <50 lbs. changes nothing to costs, paperwork, or travel hassle, so, frankly, why not