Here is the Playstation interface schematic. All diodes are standard signal (1N914) diodes:
 

To hook up the second controller duplicate all connections except DB25-10 and DB25-12. For pad 2, replace the DB25-10 connection with a connection to DB25-13, and the DB25-12 with DB25-15. Note that the parallel port may not be able to supply enough power to run a second controller, and you may only be able to run one at a time without using an external power supply.

The optional line is only necessary if you are running a Dual-Shock pad, and want to use force feedback. If you're not running a dual-shock pad, or don't want to use force feedback, don't hook anything up to the optional (pad pin 3) pin/line.

This interface has been built and tested by different folks on several analog, digital, racing, Sony(tm) and clone pads. If you're having troubles, and have already double-checked the wiring and diode orientation, try using the Advanced tab in the control panel and increasing the PSX scan delay.

If your controller doesn't power up (many racing controllers need more power than the parallel port can supply, for example) you may want to try adding an external 5V power supply:

• Remove the diodes from the DB25 connector
• Replace with the +5V lead
• Wire the supply ground line to the DB25-18/19 line.

 Direct Pad Pro 5.0
 

Once you've got the interface built, it's time to install the drivers so that Windows can communicate with it. Go to the "Control Panels" window, and select the "Game Controllers" icon.
Select the "Add..." button in the Game Controllers window, then select "Add Other..." and "Have Disk..." in the windows that appear. Specify the path into which you've extracted the .ZIP file, and hit OK.

Select "DirectPad Pro Controller" from the list (this version does not have individual drivers for each joystick type). You will then be returned to the "Add Game Controller" dialog. For the second time, select the "DirectPad Pro Controller" and you're almost done. If you're running a Dual-Shock controller and want force feedback, select the "DirectPad Pro Force Feedback Controller" from the listing, after performing the "Have Disk" operation.

Finally, double-click on the newly installed joystick. Use the dialogs to configure to the proper interface, parallel port, and controller ID (when using multiple controllers).

If you're using multiple controllers you need to repeat this exercise for as many pads as you have connected.