For slaving two motors to drive a single axis, see Help with Drivers & Stepper selection please.
For a quick and dirty way to generate 12 volts to drive another fan, you could put together a simple 7812 voltage regulator circuit (there is already one 7812 on the driver board to put out the voltage for the fan on the heatsink - it is possible that you could draw additional current from that regulator, however depending on the motor voltage that you are using, the onboard regulator is already dissipating a lot of watts, so it might be better to use a second regulator):
You should be able to get a 7812 or equivalent from any electronics hobby supplier. You will want to have a good heat sink on it. Take the input power from the motor power supply.