You don't see scopes on double barreled shotguns for the same reasons.Like double barreled rifles they are both close range weapons.

A scope magnifies for distance, so why have it on a close range weapon designed with a second barrel instantly availible for close encounters of the hair-raising kind? Scopes just aren't good for close range work!! Just doesn't quite make sense. It also affects the handling qualities of the rifle.

It is double rifle heresy but I quess in theory you could mount a non magnifying red dot sight on a double due to eyesight problems but the handling issue remains. I use them on handguns with good results. Blaser double rifles have provisions for scope nounting but you had better have a quick detachible scope to remove for close work.If you suddenly run into a close range situation with the scope on higher power you may develop critical terminal personal problems .

I must qualify this by saying I have only one double rifle but many open sighted double barreled shotguns and rifles. If I must have an optical aid of some sort non magnifying red dot would be my only consideration.

Just my 2cents---