The reason for the dual actions being - you may need to shut the engine down but have no fire thus the handle is a fuel cut-off mechanism you have to remember turbines are engines that will run or burn and turn as long as they have fuel and only have igniters or spark plugs for start and heavy weather - so shutting off the fuel is the fastest and quickest way to stop them.

Then you fire the fire suppressant or extinquisher bottles if you have a fire to put it out. If you have the latter situation you will find that it is a one shot deal, if not out you have a real problem.