Chuks,
Hmm, your company policy is to fire one bottle, wait, fire second bottle, wait, then if necessary evacuate.
I can understand their reasoning, first of all firing both bottles at once may not put out a fire, but discharging one after the other may. It also is a bad idea to have passengers going out the overwing exits if an engine is still on fire or smoldering, and they don't want passengers roaming around a runway with fire trucks racing to your aircraft, etc.
When you broadcast over the PA that we have an engine fire and to evacuate the aircraft in a calm and orderly way, via the front exits, how many passengers are going to listen beyond the words fire, and evacuate?
In a real life situation you make the decisions, if this situation occurs and everyone gets out of the aircraft without injuries, and the aircraft burns, no one will question your judgement. But if passengers are injured, killed, or burned, then every Tom, Dick and Harry will question your actions or lack of.
You have to make the best decisons based upon your training, experience, and gut feeling. Also in this situation don't hesitate to ask the tower if they see fire or smoke coming from the aircraft.
Mike