The short answer is: this is not a very good idea.
Now with OS X, this configuration is supported, and OS X will cleverly copy graphic output from one video card to another to make the system work well. You will get a fps hit when this happens.
With Vista, this configuration isn't supported. (Snarky comment: it is lame that Microsoft completely rewrote their video driver infrastructure and went backward in terms of configuration support.)
With Linux, I have no idea if this configuration can run. I do know that trying to change my configuration hosed Ubuntu thoroughly and I decided not to break my Linux boxes any more, having spent plenty of time doing that already in the last few days.
For X-Plane, we can't handle this case very well (at best you get the framerate hit) because we need to share textures between the IOS screen and main screen. So if you are trying to set up an IOS screen, you really do need a dual-headed graphics card. For what it's worth, every card I've gotten in the last few years has had two video outputs.