A double trigger served two purposes.
For the shotgun: it’s how a gentleman shoots. One shot, one bird down. No other system or selector can instantaneously select whether you’re shooting an open choke for a close bird or the right choke for a long poke. A single trigger ensures you must shoot the wrong choke missing/wounding the bird before you have access to the correct (2nd) barrel.
For the double rifle: it provides mimickry of the double shotgun. You can affordable practice with a shotgun to build instincts that help you become a better rifle shooter. That is, provided you shoot a similarly configured weapon.
Lastly: it provides resiliency. Two springs, two barrels, two triggers, two hammers, two sears, it greatly reduces the chance of a ruined hunt. While you might have half a broken gun, you still can use it as a single shot if you have a double trigger because it’s likely half the gun still works.
For the people that want a single trigger double gun or rifle, you’re welcome to do so. Their values are considerably lower so if you want to do it in a way that contravenes these benefits, you will pay a lot less. Just like you get a discount on anything that has a latent design flaw.
