I have one of those two-cartridge carriers, they do call that the 'Duo'

I like it. Although I haven't taken it out on a hunt yet, I have fired cartridges from out of it on my belt. Works well for me, I park it on my belt where I'd typically put a magazine pouch if I were carrying an autoloading pistol.
Handy and neat, the leatherwork was very nicely done -- Ratel had me send him a couple of cartridges and they made the carrier specifically for my ammunition (a nonstandard BPE round)
The price was good too!

I don't see any reason why it couldn't be run on a wrist strap or on the sling or reciever of a gun as shown in the top post.

--Tinker