Even more interesting.
Now from what I can make out you are saying that changing the bullet weight (or other things) in a SxS double can result in a change in the barrel groups on both planes, horizontal, ie shooting apart or crossed, and also vertical, ie the height of the groups in relation to the aiming point or sight picture?
And regarding the O/U rifle you are saying that there is only a change in the vertical plane and you are suggesting that therefore a correction made at the sights can put everything back to point of aim?

Well yes, it is true that USUALLY the O/U will only shift its bullet impacts along a vertical line.
But you are forgetting that the O/U is just as capable of shooting its barrels apart or crossed as the side by side is, if there is a change made to the load that the rifle was regulated for!

When changing to a different load, IF the O/U still prints both barrel groups together, but changes the height of impact, yes, you could alter the sight height to coincide with the new impact height, and all would be well.
But this OFTEN does not happen and the new load will OFTEN make the two barrel groups impact the target as two seperate groups, either apart or crossed, and as I said before, no amount of sight alteration can fix this.

Actually if any double, SxS or O/U is still putting both groups together with the new or changed load, but merely printing them too high or too low, BOTH styles, SxS and O/U can be put right with a sight height change.

If either the O/U or SxS are now shooting SEPERATE groups as a result of a load change, NEITHER O/U or SxS can be corrected with a sight height change!