Well - this is the code to control a single motor:
Note that it does have some of the code in place to control a second motor, however this shield does not seem to have the capability in place to supply the voltage for 12 VDC motors, only small 3 VDC motors. There doesn't appear to be any capability to add an external voltage source (without possibly hacking the PCB - which might be possible).
So - you would have to hack the PCB, also you would need to know what your current ratings are (running and stall) for your motors; the board uses an L293 for the h-bridge, but it can only supply so much current (it can supply the voltage, if you run it at that voltage, but the shield/board isn't designed to allow for this - plus the shield may not have the proper track widths/thicknesses to support the current needs - but you haven't stated those needs, so I can't really say for certain on that part). Finally, if the current needs can be met by the L293 - you might have to put a heatsink on the chip as well.
You would probably be better off finding another solution; perhaps something from Pololu or another vendor would work better for you.