There's the other problem. Many doubles have selective ejectors. Eject the fired case, but not the unfired round. How's that going to work if both cases are clipped to gether? Not well, I'm thinking...
The muzzle forward on the shoulder is indeed from the safari days, and the dance goes like this; The gun bearer carried the heavy double, muzzle forward on the rt shoulder, while walking ahead of the PH. The PH carried a medium game rifle (bolt action, usually), and when the double was needed, would take it from the shoulder of the bearer with his right hand, while passing his rifle forward to the gun bearer with his left. It was up to the gun bearer to get out of the line of fire, after taking the PH's rifle.
This is described in some of the books written by Taylor, Bell, and others. The PH carrying muzzle forward on the shoulder is done on a trek, not a stalk. Slings are seldome used, mostly because of their tendancy to catch on brush, always at the worst possible time. And this tendancy became well established in the era before QD sling swivels.