"One card will render a portion of the screen, the other renders the remaining. The drivers will split the screen to an approximate 50/50 based on there Nvidia's algorithm, this allows the sli setup to render up to the sum of the memories of the cards. In this case the best case scenario is 1024MB of ram available for rendering, the worst case is that one cards memory is used fully ie512MB and a portion of the remaining cards memory is used resulting in card 512+second cards memory."
Note that running in SLI mode does not double available video memory. For example, plugging in two 256 MB graphics boards still only results in a device with at most 256 MB of video memory. The reason is that the driver generally replicates all video memory across both GPUs. That is, at any given time the video memory of GPU 0 contains the same data as the video memory of GPU 1.