Texas Bill,
The "theory" that you speak of as the distance between the left and right shots being influenced by speed has to do with the time that the bullet is still in the barrel and its ultimate direction is being influenced by recoil. In other words once the powder charge is ignited and the bullet is on its way down the barrel the gun has already started recoiling back and up. Along with the upward motion the firing of the right barrel has the gun moving to the right. When shooting the left barrel, of course, it will be moving to the left. The faster the bullet is traveling the less time it is in the barrel and thus influenced by these combined motions. Graeme Wright's book "Shooting the British Double Rifle" is to me a "must have" book for the double rifle shooter as the aforementioned and much more is explained in detail.
Regards,
Buchseman
