The same way as with double rifles, Bergstutzen etc with fixed barrels.
The shooter has no way to change anything, except the sights or scope of course.
Regulation of the barrels is done by the gunmaker, using the cartridges that are agreed upon with the customer.
The customer is advised to use only these brands, loads, preferably also the same lots that were used in the regulation process. And sensibly stock up with a good quantity of these cartridges.

If you want to shoot other loads, or have purchased an old gun, it will be trial and error with factory loads, tuning by handloading, or a trip to a good gunsmith for re-regulation.

fuhrmann