Second video card (PCI) under Windows 7

"As of Vista and WDDM 1.0, the Windows Vista Display Driver Model (WDDM) doesn't support heterogenous multi-adapter configurations. If your primary video card uses the WDDM 1.0 model, then you will not be able to use the Diamond Stealth as a second display even if you can find Windows 7 compatible drivers for it. If your primary video card uses the older XPDM model, then you may be able to get your multi-adapter configuration working, but you won't be able to use Aero Glass. As @prestomation pointed out in the comments, WDDM 1.1 adds support for heterogenous multi-adapter configurations. I doubt that there are any WDDM drivers for a video card of this vintage anyway.

If your primary video card doesn't support two displays, you might want to upgrade to one that does. If it does support two displays and you're trying to add a third, you might be able to add an identical/compatible video card, given a free PCIe slot. There are also a few models of video card that support four displays on one card."

"Code 10 refers (usually) to driver error

I recommend updating the drivers, there may be a problem if one of your cards is Nvidia and the other is ATI as they do not always play nice together."