Traveling with you gun means trouble with permits, safety regulations, public transport, storage during hunting trips etc. Traveling with two guns, both a rifle and a shotgun, means more trouble, so a single gun with two or more barrel sets makes life easier on the administrative level.

I am aware that my Merkel O/U Double rifles do not really count as doubles in world of NitroX but I shoot them with much joy anyway ;-). I find that my old Merkel 201e 16/70,16/70 with an added 7x65R,7x65R barrel set handles absolutely perfect as a skeet gun and for shooting running boar.

The gun maker building the added double rifle set, has positioned the barrels so that there is no real difference when shooting with open sights.
The difference comes when shooting with the scope mounted in the SEM (Suhler Einhack Montage if I remember correctly), unfortunately age does not really help on a driven hunt in poor light conditions, so I started to use the scope on driven hunts.

I would conclude that the scope represents the compromise, not the second set of barrels.
Regards, Granhaven