Not sure what you mean by a "double shoulder holster"...for concealed carry?
Son #2 uses a shoulder rig in Afghanistan for his M9, with a triple mag holder on the off side, but it's far from "concealed" and would probably be fairly obvious in civilian garb, "mufti" if you will, except under a jacket of some sort.
For hunting, a good shoulder rig makes a lot of sense: good protection for the gun, ability to carry a long barreled revolver comfortably and still have it readily available, and easier on the hips etc than a conventional belt holster. Plus, it's out of the way of your rifle or shotgun, back pack, and other hunting paraphernalia
Here's a pic...my son is in the middle with a more or less standard armpit rig...the guy on the left has a "Tanker" type. Both of these were made there in-country, by my son. If you look closely, you can see his survival knife attached to the mag pouches. The thought being that if the airplane goes in, they'll probably only have what they're wearing if successful in getting out. Hence the equipment on their persons. The leather to build these rigs comes from Tandy, here in Louisville, who ship to them over there at cost, knowing it's going to the guys on the front lines.