Bin there. Done that.

If modifying loads and or bullets won't cut it for you then you may wish to perform adjustments.

There is a wedge between the barrel set that can be adjusted to add or remove stress between the barrels. Adding stress makes the muzzles point inwards and draws the left/right shot groupings together. Relieving stress has the opposite effect.

If the shot patterns are such that one barrel is printing higher than another, this is where things become tricky because there is no adjustment for this. You can try different loads to try to draw the shot patterns together or you can take matters into your own hands and pretend you're a middle ages Knights of the round table era blacksmith which is what I did. There is a soft brass yoke that acts as a cap on the muzzle end of the barrel set. I turned the gun upside down, rested one barrel (the one shooting high) on a block of wood and gently swat ithe yoke on the barrel side of the barrel shooting low with a ball peen hammer sufficient to ever so slightly to bend the yoke. This is not a sanctioned method to regulate this rifle and you are TOTALLY on your own in assuming the risk of breakage and/or degree of success. It did work for me but it sure felt like I was committing a crime of great magnitude in treating an expensive double rifle in this fashion. Mine was a .470NE.

Good luck and let us know what you end up doing.