Actually see this - In early 2006, NVIDIA revealed its plans for Quad SLI. When the 7900GX2 was originally demonstrated, it was with two such cards in a SLI configuration. This is possible because each GX2 has two extra SLI connectors, separate from the bridges used to link the two GPUs in one unit – one on each PCB, two per GPU, for a total of two links per GPU. When two GX2 graphics cards are installed in a SLI motherboard, these SLI connectors are
bridged using two separate SLI bridges. (In such a configuration, if the four PCBs were labeled A, B, C, D from top to bottom, A and C would be linked by an SLI bridge, as would B and D.) This way, four GPUs can contribute to performance. The 7950GX2, sold as an enthusiast-friendly card, omits the external SLI connector on one of its PCBs, meaning that only one SLI bridge is required to run two 7950GX2s in SLI.
Taken from here -
http://en.wikipedia.org/wiki/Scalable_Link_Interface