On this particular model the shotgun barrel sits inside a supporting 'loop' firmly attached to the rifle barrel.
There is a small gap around the shotgun barrel.
I literally cut a shim from a piece of a baked beans tin (it took two attempts to get the shim right), and, keeping the rolled rim on the section of tin. I then clamped one edge of this to the handle of a Sidchrome 1/2 inch drive socket handle, and used the curve, and a 16 ounce ball pein hammer, to shape the shim to match the curve of the barrel. I then tapped it in to move the end of the shotgun towards where I wanted it (up and left). The final shim was 3/8 circle, with a second small shim straight under the bottom. I tapped it in with a brass punch, and when it was correct I 'tore' the end of the shim (the rolled section) off the larger shim, and tapped it in the last little bit.
I was able to move the shotgun point of impact up and left, so the 'shot' pattern was 50/50, and a solid shot high (its a full choke, that isn't unusual), which just happened to line up with the top, centre of the ring on the scope.
On the 24C I have, I have demonstrated good accuracy, however I had the barrel reamed to a .22WMR chamber (and it is still very accurate), and the barrels shortened (due to corrosion when I bought it, beyond my ability to repair, so done by gunsmith). That has a barrel band that clamps both barrels, once I have finished doing up its stock (currently off, and part way through process) I will again align the barrels in the vertical by loosening that barrel band, and 'twisting' the barrel tips (using a large padded screwdriver), and tightening the barrel band - they were aligned before I sent it off; so I have to do that again (I was going to leave it, but it annoys me).
That one will also have the trigger honed the way I like them, whilst its apart, which improves accuracy.