While the above method is the best way, I recently did this for a customer's BurgerTime:
http://therealbobroberts.net/freeplay.html
Just make sure to keep the wire length from the coin switch to the 2-player button as short as possible and still allow the coin door to open (connect to the coin switch closest to the hinge). Otherwise, it won't work due to the resistance of the wire (at least the wire I used which was standard JAMMA harness wire)...