Traditionally, nearly all continental European two trigger over-unders, double rifles, shotguns and combinations, are set up so the front trigger fires the lower barrel. They all share the same or , at least, very similar actions and most were available on order with interchangeable barrel sets to cover all types of hunting. On break open o-us the lower barrel is closer to the hinge. So firing the bottom barrel puts less stress on the lockup of the action. That’s why the high pressure rifle barrel on combinations is usually placed below the shot tube. But even on double rifles and shotguns the “first” barrel is used far more often in hunting than the second reserve shot. So, just like side by sides are usually set up to fire front trigger = right barrel, rear = left, over unders are built front = bottom, rear = left. Additionally, double rifle and combinations usually have the primary rifle barrel trigger fitted as a single set trigger. Such a set trigger is much easier to be pushed forward for setting if it is the front one, with plenty space behind to allow for the thumb. To me, pulling the reartrigger first and then reaching out for the front one is much slower than simply letting the finger slip back after the first shot. The only real reason I know for using the rear trigger first is an old and quite worn double rifle: As the right/bottom barrel has usually seen much more use, the originally second one may be the more accurate one. Only then it makes sense to use the better barrel for a carefully aimed first shot and save the bum barrel for hasty emergency use.