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actually you can do SLI with shuttle cases starting with their latest model.
Pci_express graphics card slot and a pci+express x1 slot.


SLI needs more then just 2 PCIe slots. It needs 2 16x sized PCIe slots for the cards to fit, and motherboard support to then divide up a 16x lane into usually 2 8x lanes to each slot. PCIe slots are sized differently based on the type of card (1x, 2x, 4x, 8x and 16x), and a 16x card won't fit in a smaller slot, though a smaller card like a 4x card will fit into a 16x slot and work fine.

The motherboard maker is also responsible for providing a properly sized SLI link board that attaches to the top of both video cards.

And in a Shuttle sized box, you would also need more cooling directed at the video cards, since both cards that close togther would be a big source of heat inside the case.