The old method to do this was to use SLIpatch, but the latest and more stable and perhaps the easiest method is to use HyperSLI. The instructions are easy to follow.
- Download and install nVidia drivers
- Disable UAC on windows
- Make sure virtual machines are enabled in BIOS
- Apply HyperSLI patch
- Restart and boot with HyperSLI enabled
- Windows should detect new hardware and automatically install the driver for your second videocard
- Turn on SLI in nVidia control panel
- nVidia driver 310.70
- Asus M4A89GTD PRO/USB3
- MSI Cyclone GTX460 (geforce gtx 460)
- Zotac GTX460 (geforce gtx 460)
and have no problems so far. Benchmarking with Unreal Heaven 2.5 shows a 50% increase in performance.
Note that you will need SLI bridge for this method. Additionally, I did run into some trouble where the nVidia control panel was not showing an option for SLI. The display adapters in Device Manager also only recognized 1 of the graphics cards. To fix this problem, disable on-board graphics in BIOS and update BIOS to the latest version.