It might be easier to tell us which PSU you have. As for your question, if it takes 2 6 pin PCI-E connectors to run one card, it will take 4 to run 2. That said, most cards come with a dual molex to 6 pin PCI-E adapter.
Let us know what PSU you have and we can better answer your question. For one if your PSU only has two PCI-E connectors, there is a good chance that you won't have enough current on the 12V rail to supply two cards.