Boddington has covered this pretty well in multiple books.
Basically, if you're going with 2 different calibers, make sure there is overlap in performance.
I believe in this philosophy and use it when I hunt Africa. I’m usually always hunting DG plus PG, so I take a medium and a large bore. I’ve used a number of calibers in my (10) safaris so far, but I have pretty much leaned lately towards two favorite 2-rifle batteries; a 458 Lott & 375 H&H, or a 404 Jeffery & 9.3x62. All four calibers will take Buffalo in the event the medium needs to step in for the large bore if it goes down for whatever reason, and the two mediums will take all PG handily.
If I were anticipating longer shots like 350+ yds., I may rethink the medium bore, but this rarely happens in southern Africa where I hunt mostly now. Namibia and parts of South Africa I’ve hunted may require a long poke once in a while, but in Zim and Zambia, I don’t think I’ve shot over 250 yds. on (7) safaris there. YMMV.
