might I suggest
for true tank steering, consider two sticks, with only one axis each. this way you can have one motor in forward and one in reverse for 0 turning radius
alternately, perhaps the onboard microcontroller can be programmed as such
Y = Up X = Center, both motors forward
Y = Center X = Left, left motor reverse, right motor forward (turn left)
Y = Center X = Right, left motor forward, right motor reverse (turn right)
Y = Down X = Center, both motors reverse
edit:
or how about starting off with an easier joystick, like an electromechanical (potentiometers) one that the controller is designed to work with.