IMHO a moot, purely theoretical question,asked by people who have never regulated a dr! After reregulating about 20 double rifles, most of them side by sides, some dr drillings thrown in, about half of them by load development, the others by resoldering, and testing many more, I am quite content if I get a composite group, 3 right-left pairs, of 2 to 3" at 100 meters. This is practical hunting double rifle accuracy. The centers of most double rifle muzzles are about 1" apart, slightly more in real bigbores. Even if the individual barrels are capable of moa = 1" groups at that range and shoot true parallel this results in a 2" composite group, shaped like a liing 8. From then on the individual groups will overlap. At 200 you get at least a 3" group. If both lines of flight cross at 100 m you will get a 1" composite group at 100m and at least a 2" one at 200. Before both parallel and composite groups open up to 4" you will have exceeded the practical range of any double rifle, no matter if the barrels shoot parallel or cross at 100 m, despite of the optimistic express sights on most British rifles. Most older dr barrels will not shoot 1" individual groups at 100m at all, so the effect of shooting parallel or crossing is even less valid.
What may confuse the discussion: Even if a dr shoots perfectly parllel, the barrels have to converge slightly, more so in a side by side than in an o/u, more when shooting heavier bullet, less at high muzzle velocities. Reason: Newton's law of action and reaction. As soon as the bullet is accelerated through the barrel the gun starts to move also, unstoppable as this slight movement is taken up by the elasticity of the human skin. The gun rotates around it's center of gravity. As in a side by side the recoil force is applied to different sides of this center and on an o/u at different distances/levers from this centers, muzzles will not point at the same direction when the bullet leaves the muzzle than when the primer was ignited. For this reason all pistol barrels point downward from the line of sight (easy to check: Take any revolver and place it on a flat table upside down so that both front and rear sights touch the ssurface. Lokking from the side you will see that the barrel now points upward, away fom the table.) So the barrels have to be aligned to compensate for this invariable movement. If dr barrels were assembled perfectly parallel, perfectly possible today, it would shoot wide apart. So drs have to be regulated by cut-and try resoldering, load or screw adjustments to this day.