Very long story short, SLI enables two graphics cards to act like one big graphics card (oversimplified, but good enough for now...). NOTE that not all games or applications support this configuration...if they don't specifically support it, the game or app will essentially ignore the second card. There are SOME games and apps that support it, but the number seems to be decreasing.
As for the monitors...remember that in SLI, two cards act like one card. In that case, both of your monitors should go into the same card. You CAN plug them into their own gpu, but then it won't be true SLI.