You guys is all barking up the wrong tree. When you have a single barrel, you most likely will make that single shot count, so a second barrel just adds weight and opens that whole "regulation" can of worms. Or can of regulated worms. Or a well regulated worm-militia. Whatever.
A double barrel is best served with one rifle barrel, and one smooth bored barrel. No regulating, and you can take both large and small game. And, the gun will be significantly lighter than packing two rifle barrels around. I'd take a double barrel combination gun over a double rifle any day, unless I was hunting elephant, rhino or hippo.
If you are worried about getting a second shot on semi-dangerous game, then you load the 12 or 10guage barrel with a ball, a boat load of powder, and you should be safe at bad-breath distance. If you are worried that you'll need a follow up shot on deer or elk, or moose, I suggest you spend more time target practicing with the gun. Learn not to flinch, and choose your shots well. If you blow the first shot on a standing animal, how are you going to hit a running animal with your second shot?