Totally possible, and surprisingly easy. Install the NVidia driver, then the ATI driver (installing the ATI driver first can cause CGMiner to not detect the ATI card properly). Specify devices appropriately in your configs, and you're good to go.

I run the displays on the NVidia card, so YMMV on that point. Also, note that PhysX will be disabled by the NVidia driver when you install the ATI driver - NVidia didn't take so kindly to people trying to run PhysX on an ATI card with some hacked up drivers a while ago, and their stance hasn't changed since. Afterburner/GPUTweak/Precision-X won't work properly with the ATI card (ranges from blank information on Afterburner to Precision-X program crashes), but you can get info from GPU-Z.