Taking the last point first, they are fascinating - but one should keep in mind that it is no coincidence that 'fascination' shares a significant number of letters with 'frustration'.There are two schools of thought on regulation
1: Regulated at a specific distance (ie 50, 70 or 100 yrds etc.)
2: Both shots parallel to infinity (or perpendicular in the case of an over and under. )
My 8 x 60 prints two shots the same distance apart at 70 yrds as the distance between the centres of the bore at the muzzle which if how I prefer things.
That is with a scope, without the scope they open up a bit due to the lack of inertia the scope weight adds to the twisting movement of the recoil (mine is a side by side) but nothing drastic enough to make any odds at any distance that I would use the open sights at.
Anyway, enjoy your Double Rifle , they are fascinating...
On the matter of regulation, I can understand how a rifle should be regulated to shoot both barrels together with a particular load at a particular distance: the barrels have to be adjusted so that the centre-lines of the bores cross to just the right extent that the desired result is achieved - as I say, specifically to a particular load and range (and sight, of course).
It is less-clear to me how both shots can be parallel to infinity. Is this really possible, given the dynamics of the rifle during firing?
If your 8x60 shoots the groups the same distance apart as the muzzles at 70yds, where is the crossover? Presumably if at that range the right barrel shoots right and the left left, then it's regulated slightly further away than 70yds. If the other way round, slightly nearer? Having said that, depending on the group sizes of the two barrels relative to the distance between the muzzles, that might well just count as regulated at 70yds?