One thing to consider if you fire both firex cylinders at the same time on the ground:
You only have two shots to put the fire out. If you fire the first cylinder and some time later the fire reignites, you have a second shot to put the fire out. What would you do if you have already fired both cylinders? If you have ever put a fire out with a fire extinguisher you will be aware that secondary flare up can occur if fuel and a source of ignition remain. I would be hanging out for the 30 seconds to see whether the first shot had worked before using up my last chance.