I would think something of this sort would work to do it, though you would need a full tower, since both of the cards would stick out from under the board and it would be interesting to cool them.
Usually the motherboards with 3 and above pci-express slots for graphics cards have the room for them, they are 2.5 slot wide, look much bigger than what they are.
the asus card is actually only 2.5 slots so it will go back to back on most motherboards in Sli, but honestly the dc2 models are good enough and only take up the standard 2 slots