You can always modify your joystick in order to have two separated fire buttons but in order to do that you need a cable with 9 wires (actually even 7 would be enough but it is difficult to find a cable with exactly the 7 wires you need). 7 wires are for directions, ground, fire1 an fire2. The remaining 2 wires are for +5v (useful for autofire circuits) and fire3 (unused). On the Internet there are lot of schematics like this one:
