Several questions in other posts prompted me to write this.
I have been using my 3 doubles for 6 to 7 years. I have used them for game from coyote, deer, pigs, caribou, bear, plains game, buff and elephant.
I have done quite a bit of shooting and experimentation with different loads and bullets.
These are some of my findings.
With a little work... sometimes not much at all, I have been able to get several different bullets to hit with the sights and regulate to the rifle. I have found all the talk about doubles being a nightmare to load for are completely false. I have had some bolt rifles that were way more "finicky" than my doubles.
Example; in my 450 No2, the 350 Hornady, 480 Woodleigh soft and solid, 500 Hornady s&s, and the 500 Swift A- Frame all hit together to 100 yards on paper.
Same with 300gr Hawks, 400gr Hawks, 300 and 400 gr Barnes X bullets, 400gr Woodleigh soft and solids in my 450/400 3 1/4".
In the 9,3x74R I can use 285 Hawk, 286 Woodleigh soft, and 286 Nosler Partition out to 200 yards with the same scope setting. At 300 yards the round nose bullets drop a few inches below the pointed Nosler. Also I discovered that at 100 yards with a Woodleigh soft in the right bbl and a Nosler Partition in the left my R/L group is a little snaller that if I shoot 2 Woodleigh's or 2 Noslers.
I think being able to have 2 different bullets in your double at the same time is one of the advantages of the double. Not only soft and solid when in Africa, but many times when hunting North America I carry a "soft" soft in the right bbl and a "harder" soft in the left. This has worked well for me several times. Sometimes when hunting deer,pigs and turkey at the same time I put a soft in the right and a solid in the left. Inside of 30 yards it does not make any difference which bbl I fire first, at 100 yards it does, I must shoot the right bbl first to get proper impact points with the 9,3.
Also being able to silently change from one ammo type to another is a good feature, as anytime we encountered elephant while hunting plains game the PH had me change to 2 solids while we avoided the elephants.
When in Africa on the buttstock of my 450 No2 I carried only solids. On the 9,3x74R the last 4 rounds were solids. I had extra softs and solids on my belt.
I always put the softs in front and the solids in back, thus there is no problem finding the one you need.
When under a time frame I load off the buttstock, and refill it as necessary.
After using my doubles almost exclusively for the past several years I consider the double rifle far superior to any other rifle as a hunting gun, at least to 250-300 yards.
That immediate second shot is very handy.

If I had the money I would get a 2 barreled set from Chapuis, a 375H&H and a 300 Win Mag. Now I could really put some theories to the test with that combo.