ok, that makes sense. I just wasn't sure exactly what you were trying to accomplish. wiring up the external gamepad ports shouldnt be too bad. If I were going to do that, i would use a 25pin manual switch box to do this. I don't know how many wires there are in a Playstation controller cord, you may get away with a 9pin switchbox. this is how i did with my x-arcade to switch it between my PC and my Dreamcast. it is a little different than what you are doing, but the same theory applies. the switch basically has 1 input and 2 (or more) out puts. you would need some playstation extension cables, and some 25 or 9 pin cables to splice together. you would connect the input port on the switch to the console. one of the outputs would go to the arcade stick, the other to the gameport. then you would have to turn the knob to switch between them. you would have to use two separate switches, one for each player, so that one player can use the arcade stick, and the other a gamepad. otherwise you would both have to use the same type of control.
unless there is some kind of other way that I am not aware of, this is also how you would switch your controls between the PC, and the playstation.