That interface will let you hook up joysticks to the pc, it won't work with the xbox.
The Hori ex2 is an arcade joystick that uses knockoff seimitsu parts, since its already wired to joysticks and buttons, you wouldn't have to hack a xbox pad, you could either build two into a cp, or transplant the sticks and buttons onto a cp, or just use the guts with your own buttons and sticks.
Not sure of a way to wire something to a pc and xbox at the same time, with the same controller. The only thing I can think of would be to wire the microswitches in your controls to an to sets of inputs, one for the pc and one for the xbox.
Maybe you could take that paradise arcade and hook it up to your pc. Take 2 hori sticks and connect them to your xbox an then connect the paradise arcade controller to the hori sticks. that way when you use the hori sticks, it sends out 2 sets of inputs, one to the xbox and one to the pc.