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
Since the cards are two-slot widths, they expect that you'd use slots 1 and 3, and the included SLI bridge should accommodate that. If you want to use a wider spacing, you can get longer SLI bridges from nVidia: