Windows 7 does not allow use of onboard graphics at the same time as a video card without Hydra (which you motherboard does not support).
I asked a similar question: Is it better to add a second video card in Crossfire or replace an aging video card?.
It looks like your motherboard does not support SLI/Crossfire. You're stuck having to buy a new graphics card. Sorry.
UPDATE
You will need to buy a graphics card that has three outputs which are generally a dual PCI slot cards. Unless you get a card that has displayPort in it.
First off, check the output ports on one graphics card - it depends on the manufacturer. If I could guess, I'd say any high-end card right now would have at least three ports, but whether or not those ports are the right ones is up to you.
There are variables to consider.
Price:
Usually, a twin-card setup is far more expensive than a single-card setup. But because you are comparing the 770 to the 780 Ti, that is not the case here.
During its introduction the GeForce GTX 780 Ti is second most expensive card in NVidia's line-up. I'm not sure if the price has gone down today, but I do know that the 770 received some price cuts. You might be able to buy two 770's, for the price of one 780 Ti.
Performance:
Here is where things take a dip. SLI's performance doesn't actually do that well for its price, graphics quality wise - you will still be held back by the individual cards' weaknesses. If the 770 can't turn on X feature in Y game, it unlikely that adding another similar card will address that.
Where SLI shines is with the use of multiple monitors - SLI will increase your Frames Per Second (though it will never double it).
For comparison, the 780 Ti can't do 4k resolutions at 60fps. As multiple monitors can be treated as a extra-wide monitor with an incredibly high resolution, certain setups might choke even the 780 Ti.
Power Consumption:
No brainer. Twin 770's are sure to bring your bill up, as well as your noise levels. Might be something to consider.
Conclusion:
The 780 Ti is a beast, and in standard clock settings, is the fastest single graphics card that I know of. The 770 is no slouch, but individually, it cannot match the 780 Ti. However, two 770's, while still unable to match the 780 Ti, might be cheaper, and will likely produce better FPS's over multiple monitors.
Best Answer
Apparently the iGPU Multi-Monitor setting under advanced/system agent configuration/graphics configuration needs to be turned on. This is labelled for being for virtu but is necessary for the integrated GPU to be detectable by windows, even if you arn't running virtu.
Obviously the location of this may vary with different motherboard brands and the name of the setting may be different.
My guess is since lucid logix uses both GPUs, it tells the motherboard to simply run both - independent of whether you're running more than one monitor (with lucid outputting what you're rendering on the dedicated GPU to the integrated GPU), or using a multi-monitor set up taking advantage of both the IGPU and Dedicated GPU as independant outputs without lucid.
This might changed with later boards - but it was made possible by on die GPUs not using up PCIe lanes, and newer version of windows playing nice with more than one video card driver at a time.