@Uvuvwevwevwe-Onyetenyevwe-Ugwemubwem-Ossas you can do this only by writing custom script.
There's an example how would you do this:
Requires Core.lib to work properly (put this into onUpdate)
This way would work dope as if you just hold these two keys Fly module would toggle on and instantly toggle off. Be careful: I specified Right Alt key.
For proper functional you need to use onKey. Maybe learn some ScriptAPI?
EDIT: to be not boring there's an example of using onKey:
onKey: function(e) { e.key == Keyboard.KEY_RMENU && e.key == KEY_1 && FlyModule.toggle(); }