Not sure if this will answer your question....
As long as you understand how a pushbutton/joystick works, using a coin door/box/whatever works the same; they all use microswitches connected to a keyboard encoder that translates the push of a pushbutton into a keyboard keystroke.
To re-iterate, when you push a button, or press a direction on a joystick, it basically is turned into a simple keyboard keystroke. When you drop a coin in, it translates into a keystroke.
So, you just have to wire up the coin door microswitches to translate into keystrokes "1" and "2" since MAME uses these for coin inputs (via keyboard encoder which I think your xarcade has in it).
Hope I didn't misunderstand your question....