You only need one bridge, a second will offer no adavantage at all. Thats one of the first FAQ on the Nvidia SLI site.
cool thanks that helps alot. so no need to buy another.
in fact just for fun install both cards with no bridge, run your favorite bench mark, then install the bridge reboot and run the bench again. You will see only like a 1 to 5% increase in SLI. The 470s and 480s can not saturate the pcie 2.0 bus so the 2 cards can communicate through the motherboard and run sli with no bridge and with almost no loss in performance.
edit do not put the bridge inplace with the computer on... power down first.