bonanza, like Mick says, this should not happen if all is well with the rifle and the loads and the shooting hold.

There has been this myth about the bullets starting off at the barrel spacing width and then reaching a certain distance which the myth perpertrators call "the regulation distance", then at that distance the shots cross and will again be the barrel spacing width crossed at twice the "regulation distance"!

Total crap!

If you shone two lazer beams out of the muzzle centers they would maybe do what I've just described above, but this is NOT what happens to the bullet paths of a correctly regulated double shooting correct loads and being shot correctly!

Graeme Wright has a very good description on bullet paths of doubles in his "Shooting The British Double Rifle" book.
This is a must have book for double owners IMO.