It is easy to add a second joystick to the IBM PC.  The standard
female connector on the IBM PC's game port is a 15 pin connector.
A joystick only requires 7 lines to run.  Your first joystick
uses the first 8 pins of the 15.  You can add a second joystick
on the last 7 pins, utilizing all 15 pins on the connector.

You can make yourself a Y cable (male 15-pin to 2 female 15-pin
connectors) running from the game port to the two joysticks.
Connect the pins as follows:


     Game adapter       Joystick  #1       Joystick #2
          Pin                Pin                Pin

          1                   1
          2                   2
          3                   3
          4                   4
          5                   5
          6                   6
          7                   7
          8                   8
          9                                      1
          10                                     2
          11                                     3
          12                                     4 and 5
          13                                     6
          14                                     7
          15                                     8


This works great with Flight Simulator, F15 Strike Eagle, and
Larry Bird & Julius Irving Go One on One.                                                                     