I've replied to this rules question in the original topic, but Magnus, as always, has made the point more eloquently than me. Whether you're playing second edition (like me) or NetEpic, combined barrages can only be conducted by the same detachment (i.e. the models will be the same). The Artillery Company has different detachments, so you must resolve their barrages separately.
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SoƱando con una playa donde brilla el sol, un arco iris ilumina el cielo, y el mar espejea iridescentemente.