First of all, make sure you have enabeled the second screen in XP/Vista/W7.
Then make sure you do NOT use in windowed mode, then it will not work.
Check your FSX config to be sure you have both screens avialible.
Mine looks like this:
[DISPLAY.Device.NVIDIA GeForce GTX 275.0.0]
[DISPLAY.Device.NVIDIA GeForce GTX 275.0.1]
My setup has different resolution on the screens so to make it easy I set the lower resolution screen as primary monitor (this is the lower one)
Start off with the 2D cockpit. You can now drag and drop the "world outside the cockpit" on to the second monitor.
Then just right click and choose VC cocpit and it will snap in to full screen resolution. Presto....you now have all 2D on the lower while still
enjoying 3D VC on the other screen. The benefits of doing this is that all popups will still be comming to the lower screen and not into the VC.
Another way is of course to start in 3D VC, press Shift+1 and the 2D will pop up. Just drag it to the lower screen and you are done.
The bad thing about this that you need to re-size the 2D cockpit to fit the resolution..unless you have the same as the monitor above of course, then it's no problems.
Then you have all popups (throttle, GPS and stuff like that ) that will start from the 3D VC. You need to drag them down to the lower too...and re-size. So you understand why I use the first method.
There is no loss of fps using either method, so I'm very happy with it.