You don't need additional electronics to use Fire2 and Fire3 on both joysticks, you only have to use the same A6 pin on the two joystick ports and use the K or L lines(B2 or A2) as commons, instead of the J line(A1).
I don't know why Pear didn't add the extra Fire buttons on his cheap adapter, but it is only a mater of solder 4 leads on it.
Ah, now I realise, sorry, not for a standard 9 lead joystick cable, but you can tailor one arcade joystick connected directly to the EDGE on the Enterprise. Then the cable of the joystick also will need 9 leads, because ground and +5v are not used.