SLI works like so. The first card splits the work in half, sending the other half off to the other gpu via the bridge or chipset, then once processed, the gpu sends it back across the bridge where the second card compiles it and sends it off to join the other data being processed by the cpu and other components. Because of that, the first GPU will always be hotter and be working harder with more memory used because it has to store the data the second card sends back.