I think your motherboard may disable onboard graphics when there is a discrete card installed. But... maybe not.
Also, what OS are you using? Some early versions, like Vista and possibly XP could install only one graphics driver. Your motherboard and Nvidia will be two different ones. Windows 7 is OK with two different graphics drivers. I have tested that.
Take one of your monitors and install it in the onboard adapter and see if you can get it to work with the remaining monitor in the discrete card. If you are successful, adding a second monitor to your discrete card should be successful.
As an alternative, get a cheap second video card. Any Nvidia series 6xxx or higher card will work.
It won't work.
I was hoping for AMD to release an integrated 5200 with 3 screen support, but sadly they didn't and went with a 4250 instead.
It wouldn't actually help in your case, besides you being able to switch between the two.
Otherwise, you'd need either an adapter or a second 460, both in which are rather expensive.