WCF
I did a similar project years ago and got excellent results, 2.5-3 inch groups at 50 yards. First, do your barrels shoot to their respective sides? Right to the right, left to the left? If so you can bring them together by shortening the barrels (read hacksaw). This reduces the time the slug spends in the barrel as it recoils up and away from center. The effect is the same as if you increase velocity in a double rifle to bring the groups together. I cut my barrels down an inch at a time until I got overlapping groups. This presumes decent sights and a load that shoots well in the gun. On mine I used a Williams sight drilled and tapped onto the rib and a ramp front sight superglued to the rib about 4 inches from the muzzles. I wasn't worried about where they shot, just how they grouped at that point. When I got the overlapping groups I wanted I cleaned it up, correctly installed the front sight and adjusted the rear to get thr POI about 3 inches high at 50 yds. BTW it was great fun to shoot a couple of shots on the public range then whip out the hacksaw and lop off a chunk of barrels and try again. Drew some interedting comments until they started to understand the method to my obvious madness.
On the other hand if the barrels cross ingnore all of the above and just put the sights of your choise on and load some slugs a little slower or use a heavier slug to increase travel time in the barrels and or increase recoil to uncross the groups.
Have fun!
Best.