I thought cross-over (divergence) was deliberately built in to doubles to counter the effct that recoil has on each barrel ie left barrel is left of the centreline so will recoil towards that direction. So the barrels are set with a slight crossover. It's still a matter of getting the crossover correct.
Barrels on doubles are set to converge slightly, to counteract the effect of recoil on POI. With a SxS, the right barrel, being slightly to the right of the centreline of the butt, as well as being above the centre of the buttplate, tends to recoil up and to the right, while the left barrel tends to move up and to the left. This movement is happening as the shot charge is moving down the barrel.
This is part of the reason why testing for POI should really be done by holding the gun as I said earlier - and ideally by the person who'll be shooting it - so that it recoils the same way as it will when shooting in the field. It is also why it is worth testing a few different cartridges, because different combinations of velocity and recoil can also have an effect.Of course all this is usually a lot less critical than it is in a double rifle, as the spread of shot makes up for a fair bit of error - that is where Bruce Buck's advice (above) comes into play.
