DB - you may well be right, or should I say correct to avoid confusion, about SBS. Point stands about stack-barrel guns tho! I don't have much experience with smooth-bores, but I always assumed that you fired the more open choke barrel, the first one set to fire, at the closer bird, and the tighter choke at the further one.
I know a very rude joke about this, but p'raps not at Xmas!