I know your ML is a shotgun and not a rifle - so who knows how or even if it was regulated to converge at one spot. To boot - you don't have any rifling.
I've owned 1/2 dozen original British BP double rifles over the years - now only 2, both cased and 1 is RB double (actually a belted ball). Whether BL or ML you can find the regulation (which is part of the fun) by bullet weight (conicals) and powder charge - yes POI will move. Slowing down will actually cause the projectile to stay in the bore longer; i.e. right bbl will go right & left to left. Faster will be the opposite.
I wrote and article for the NMLRA MuzzleBlast a few years ago, that was published Nov. 2107 titled "Three Deuces" - all 45 fast twist double rifles. Alexander Henry, John Rigby and a Whitworth.
I'll be curious to your testing as ML double rifles are nothing new - been around for 200 years +/-.