If I remember correctly, BRNO used to make an O/U DR/combination gun/shotgun with a double trigger mechanism (ZH 300). However, if you pulled the rear trigger twice, both barrels fired, making it, in effect, a single trigger. It would seem fairly simple to apply the same principle to the front trigger, giving the shooter an instant choice of which barrel fired first, without having to change triggers between shots.

I have no idea how complicated and fragile this mechanism was, but it's at least food for thought.