The fascinating thing about regulating a scoped Double is you may ideally wish the left barrel slightly to the left of the scopes crosshairs at distance and the right barrel slightly to the right!! Two side by side POI with one approximate crosshair point for both!Wild vs a bolt where the crosshairs are directly on the POI at distance!!!

With an O/U in theory I guess you would want one bullet POI hitting over and one hitting under your crosshairs at distance!! Weird!
Maybe better to shoot the lower barrel first at distance per dnovo's suggestion??

Sorry for all the posts on my own thread--just thinking out loud!!