Check out this article. It seems like what you ask for (and I have been wanting for some time now too) will be possible when drivers are released in September.
Apparently this has always been possible, but nVidia wouldn't allow it and are now finally releasing drivers to do so. I believe you can only do it if you have a recent card (8xxx, 9xxx, GTX 2xx).
So, the answer is No until September.http://www.tomshardware.com/news/nvidia-driver-big-bang-geforce,6039.html