I usually shoot the lower barrel of my O/U first as I perceive less recoil and am able to get a second shot off faster.

Does this phenomena-one barrel seem to kick less-occur with SxS shotguns also?

Most s/b/s guns with double triggers fire the RIGHT barrel first off the FRONT TRIGGER of the two triggers.

I guess that as the gunsmith or gunmaker cold have easily have swapped the order so that the FRONT TRIGGER fired the LEFT barrel (although the triggers would have had to have been made that way around to do it) that the right barrel must disturb the position of the gun less?

However I, like others here in UK, was taught to make a clear (even though almost un-noticed) second mount of the gun for then second shot.

Right bbl. for a right handed shooter is supposed to help pull gun away from the face which may reduce recoil effect; I have never really noticed it. Maybe with the really large bore double rifles the effect is more pronounced. On an O/U the lower bbl. is closer to the stock centerline so the recoil should be in a more straight back direction, the upper bbl will tend to rise a bit more at the shot. There are so many other variables that can over shadow any of this.
C.G.B.