[X3AP] Trouble with assigning all axis options to CH combined joystick

[HalcyonSpirit]

X3: Albion Prelude (GOG Terran War Pack Version 3.1a, English)

DXDiag Report: http://pastebin.com/jAVh1tGd

I'm currently using the CH Products Fighterstick, Pro Throttle and Pro Pedals. From what I understand, the game isn't able to use more than one joystick device, so all three are mapped into a single device using the CH Control Manager. However, I'm having trouble getting the game to recognize all the available axes.

As shown here in the game, there are six available axes in the controls setup. The Throttle (Pro Throttle Z axis), Rudder (Pro Pedals R axis), and Joystick 1 X/Y axes (Fighterstick X/Y axes) all work fine, but I can't get the Joystick 2 X/Y axes to work. I'd like to use those two via the thumbstick on the Pro Throttle, but it seems like none of the remaining axis options in the Control Manager (U, V, Slider 0, Slider 1) are recognized by the game.

Does anyone know what the game is looking at for those axes, and how to assign them? Is there a limit to how many axes are recognized by the game?