John - you are right, of course - vertical dispersion with the overunder. I had side by side ingrained in my mind, even though I knew we were talking overunder. They are already on the same horizontal plane and if shooting parallel, should have individual barrel points of impact spread vertically as well - an instant 1" + height (or so) given to the top barrel's bullet. Crossing would have the top barrel shooting low and the bottom barrel shooting high.
The same 'rules' would apply though, as do with side by sides for spreading or bringing the groups together.