It can be done, but should really, never, ever, ever be used for critical viewing – for sync or for color. Seriously.
Once the Mac OS sees both monitors, and both are lit up with the Mac OS desktop, launch FCP.
VIEW Menu—> Video Playback. (at the bottom)
You’ll see a few settings, including:
Digital Cinema Desktop Preview
and
Digital Cinema Desktop Preview Main
These correspond to your left and right monitor. Each one of these correspond to your left and right monitor, and will change depending on what your ‘Main’ monitor is.
Also, make sure the EXTERNAL VIDEO is set to ALL FRAMES.
~Michael
.: michael kammes mpse
.: senior applications editor . post workflow consultant
.: audio specialist . act fcp . acsr
.: michaelkammes.com