Tactical Shotgun Scabbards are here:
More info by our pal MM here (You have to find it on this page): http://www.militarymorons.com/weapons/longguns.html
SOTech Version that works with MOLLE called teh "Breachers Manual Tool Quiver" here:
Now the prob with your idea is the guns you want to scabbard mount have external magazines. The external mags will always cause you a hangup prob expecially when back mounted. From back to the cowboy Winchester scabbards to the Garand Indian Motorcycle and Jeep scabbards, these worked because they weren't person mounted. I can see this being possible with a G36c, but an M14, even a SOCOM you'd need to have quite long arms or design it with a velcor side breakaway.
I do have this Thompson shoulder slung scabbard though:
But in airsoft you can upgrade a G36 to out shoot a stock M14. I don't see a real reason to carry an M14 and a 2nd smaller AEG. If it were a Bolt Sniper + AEG that makes sense.