Good on ya, for actually testing what loads work. A couple things....
First, while Rottweil Brenekkes kick very well, the 20 gauge version is up with the 12 as to effect on whitetails,IME.
My guess is at longer ranges, the groups will show the left barrel shooting to the right of the right barrel. IOW, they will cross.
When double rifles are regulated, it's at a given distance. Since most double rifles these days are meant to be stoppers on dangerous game, that's usually close work. I think H&H regulated stopping rifles at 40 yards and zeroed the open sights in for that on the first leaf.
What sights are you using?