Enabling a player's jets can only be done on the client - but you're wanting to have this double-spacebar-to-jet option available on a per-player-datablock basis. You're mixing client and serverside stuff so it won't really work.
Why do you need an odd key combination to enable jets, anyway? It's more understandable if you use the same type of jets everything else has...
but if you double tapped WASD/Spacebar, it'd give you a jet boost.
You could use brick movement controls. o: