You're right, any card with two 'outs' can do that, but thats not the primary feature of SLI. Infact technically its quite the opposite - to utilize 2 cards into a single output.
You'd have two cards, thats twice as much punch!
Nope... You don't just multiply by two. The only time it is good to get 2 cards in Sli over getting one more powerful one is when you've got the best cards on the market and the only way to improve is another of the same card.
The short answer is it increases your Graphics processing power & is nothing to do with dual display (not primarily anyway).
Nvidia use SLI, ATI use crossfire ... its only a name but the concept is the same.
Its a huge topic with many opinions and views, whats best, is it worth it, yada yada yada. Do a google search on SLI versus crossfire, by the time you've read over all the arguments you'll be well up to speed on utilizing two cards.