Don't think it cocks the second bbl on recoil , you cock both hammers on opening or closing depending on gun. You reset the trigger for the second bbl from inerta (sp) during fireing . Its a safety to keep both bbls from fireing at the same time. If it cocked on fireing how would a selective trigger work ? Some guns have mechanical triggers instead of inerta (sp) that don't need the rap to reset .
Not trying to be a SA just offer more to explain.