I have put together a controller that can operate as Player one and Player two using one single connection on the GX4000, great for those converted titles that need the use of the second controller.
I use a Chinese after market SNES controller and utilize all 8 buttons.
When Plugged into Joystick Port 1
DIRECTION = 1UP Normal
A RED = 1UP FIRE 1
B YELLOW = 1UP FIRE 2
START = 2UP UP
SELECT = 2UP DOWN
REAR LEFT = 2UP LEFT
REAR RIGHT = 2UP RIGHT
X BLUE = 2UP FIRE 1
Y GREEN = 2UP FIRE 2
When plugged into Joystick port 2 it acts as a a normal Player two controller.
At present its still a work in progress :) but so far I am very happy with the results.
in this video, I demonstrated the controller in the game Crafton Xunk which requires two controllers.
In this video it shows the button presses in use in Basic.
And its very hard to hold a phone and video it :D