Just something to think about.
The 360 pads are recognized as joysticks by a PC if you download the drivers from microsoft. That means that you could use the CP for both the Xbox and a PC. You can get a KVM - Keyboard Video Mouse - switch to switch between the two. Its basically a box that was originally used to allow people to use the same keyboard, mouse and monitor for 2 different computers. You hook the box up the PC1 and PC2, then you hook the keyboard, video and mouse into the box. Then you either run a script, or push a button and you can control either pc with just one set of inputs. Now, they have KVM's with USB ports and sound ports so you can get one, plug your controls into it, your speakers into it, your monitor into it, then plug your xbox into it and your PC into it. You boot up your cab, its a MAME PC, then, say, you want to play the xbox. You push a button, everything switches over. You hit a guide button on your cp to turn the xbox on. Voila. Mame PC and Xbox, in the same cab, controlled by the same controllers.
You dig?
That's what I am building
OK. Why not sell the panel as is, and use the money to build a 2 player panel with 2 hacked 360 pads? You can probably get a couple hundred bucks for the thing, a 2P CP will cost maybe 100 bucks to build TOPS.
Anywhow, back to what you were asking. All you need is the appropriate resistor that will lower the voltage from 12v to 5v. Just do a google search for "resistor 12v to 5v" there are resistor calculators all over the web. Then head over to radioshack and buy a couple. They are cheap. Then solder the resistors onto the positive leads of your joystick.