The faster they are going, the lower they will strike.

I'm surprised they are holding at 8", though.

I would continue with load developement, especially with longshot for higher speed. You should be able to get it up to about 1,700fps and that might being the barrels in - or separate them further - either/or but at least you'll know what is needed.

Sounds like great grouping, individually - marvelous.

Wads, ball diameter and powder. All combine to regulation.

Now, with longshot grouping at 8" - centre to centre or overall - side to side? The individual groups must be about 5" to 6" apart, on centres. The spread is taken centre of group to centre of group.

I'd try some more load development - powder charge changes and maybe wads - I ran my 12 bore smoothie up to only 1,550fps, but it shot into the same group at 50yards and I was able to hold both barrels on a 10" plate, offhand at 100 meters.