A long time ago when I used to play around with computer electronics I recall that the PC gameport was limited to 4 analogue inputs and 4 digital inputs.
This meant that you could connect two simple devices each with two axes and 2 buttons.
To connect two such controllers you needed a specailly wired Y-adaptor.
Then came more advanced joysticks that used all of the analogue inputs (e.g. elevator, aileron, rudder and throttle) and they also used all 4 digtal buttons. If you had one of these joysticks connected you couldn't connect a second.
I understand that the support of more than 4 buttons and hats is done by actually by setting combinations of the 4 digital inputs at the same time.
That is where my knowledge ends.
What I don't know is how the USB controllers work. Has the architecture changed or are they still mapped onto the limited Game Port?
My son's sidewinder has one D-Pad and about 10 buttons so it must be using two analogue inputs and all four digital inputs.
This would suggest that I could not connect another pad using the Game Port (even if I had a Y-Lead) but what about a USB pad?