For a simple joystick doubler, you need two 9-way D plugs, and one 9-way D socket. Simply connect pins 1, 2, 3, 4, and 6 on the socket to exactly the same pins on BOTH plugs. Then connect pin 8 on the socket to pin 8 on the plug for joystick#1, and pin 9 on the socket to pin 8 on the plug for joystick#2 (NOT pin 9, notice). And that's it.
HOWEVER! You will find that you get joysticks clashing with each other - if one joystick is pushing up-left and the other joystick is pushing up-right for example, then it appears that BOTH joysticks are pushing up, left and right all at the same time. How can you solve this? There are two methods: firstly you can try adding 5 diodes to each D plug: connect the anode to the direction pin on the joystick port (the socket) and the cathode to the relevant direction pin on the plug. Do this for every direction pin, and connect pins 8 and 9 on the joystick port as before. I've had limited success with this - one port worked but the other was temperamental. It might be better to use germanium diodes rather than silicon diodes since the voltage drop is lower. A far better solution is method two: use two hex tristate buffers with /OE fed from STROBEL1 or STROBEL2 (pins 8 and 9) on the joystick port. I shan't explain how to do it here, but if you're brave enough to give it a go then you can probably figure it out for yourself. Perhaps I'll post the circuit diagram up somewhere... (pin 5 is Ov, and pin 7 is +5v)