Whats better, two 2gb graphics cards, or one 4gb graphics card?

I'm buying a new pc and I'm having trouble with graphics cards. I can afford two EVGA Liquid Cooling NVIDIA GTX 680 Hydro Copper 2GB VRAM, or one EVGA Liquid Cooling NVIDIA GTX 690 4GB Hydro Copper VRAM and an NVIDIA GeForce GTX 550 Ti 2GB 16X PCIe Video Card. which combination is better?
Update: If it helps any, the pc is for extreme gaming, about 14 hours a day and up.
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