It is possible, but only with the 600 series. Before the 600 series, 2D Surround required SLI.
You can create an extended display using the 2 cards, but not in 2D Surround, which is required for gaming across 3 screens.. I have, however, heard of some 3rd party software that may allow this, but I do not know the name of it. It's also not free.