Büchsemann,
As in almost all drillings, except mine, the front trigger fires the rifle barrel if the switch is on "Kugel" or the right shot one if "Schrot" is selected, the rear/left trigger always serving the left shot barrel regardless of the selector position, it is simply convenient to load the slug in the left barrel. This allows you to fire two quick "left and right" shots on f.i. boar without having to manipulate a switch in between. It is also convenient to have an insert barrel inside the right shot barrel, as this enables you to use the set trigger for a deliberate shot at some small game. So a really good gunmaker regulates a drilling this way: 1- the shot barrels have to shoot into a usable left-right shot pattern over the open top rib. 2- the rifle barrel and the left shot barrel should shoot both the bullet and the slug to the same poi at slug range, up to 50 meters, over the automatic open sight. Now, where the right shot barrel shoots with slugs, is of minor consequence, as the open rear sight will be down and useless anyhow when the right barrel is selected. So, if the gunmaker was really good, slugs from the left shotgun barrel are more likely to shoot to your open sights.
BTW, regarding double rifle drillings: As you intend to use slugs in the shot barrel of your Bemesu dr-drilling, thoroughly check point of impact before using it on game. You already have two rifle barrels on hand, so the lower barrel was intended for shot use only, in case some shootable fur and feather offers an opportunity. A dr-drilling usually has no rear sight folding out of sight automatic. So many of them are regulated to throw a hasty shot pattern above the line of open rifle sights. At least my Lovena dr-drilling does this. Consequently it shoots a slug high too. As Bernhard Merkel was an avid hunter besides one of the best gunmakers, he likely regulated your drilling this way also to make the shot barrel really usable.