Well, an uninformed mind I guess.

Intuitively it seems that the horizontal regulation of a pair of stacked barrels would be easier than the horizontal regulation of two barrels laid side-by-side. If different loads only changed the vertical POI (from a pair of O/U barrels) then it would seem a matter of adjusting the elevation of sights or scope.

On the other hand, it would seem that changing bullet weight and/or velocity in a side-by-side would affect both the vertical POI as well as the horizontal POI of each barrel.

Can you clarify & correct?