Yes, you put both 8 AND 6 pin connectors to card. PCI Express card slot supplies 75W, 8 pin connector 150W and 6 pin connector 75W. Card wants more than 225W, so 6+8 pin is needed.
One 2-way SLI is all that is needed fort 2 cards SLI yes. That "second" bridge would be useless IMO.