Yes that the point of having sli mobo The pci-e slot for taking on the graphics cards are earmarked you can put in one graphics card first and run the system. The slot runs at 16x then and once you get another graphics card you flip the sli selector and addin another graphics card now both cards as one. However note that not all graphics cards are fit for the sli setup. There is a list that is given by the vendor of the card manufacturer saying if that card is fit for sli functioning.
Also note that sli has to be enabled in the hardware (physically flipping a small card in asus).