Are you trying to get the shotgun to shoot to the sights?
If you just want the barrels to shoot the same place you can put a wedge between the barrels about 1/2 the way that puts pressure upward on the center of the rifle barrel. That should lower its POI. It doesn't take much pressure to change it substantially. The Valmets have a movable wedge at that point, but the rifle barrel is below the shotgun, so the opposite adjustment applies.