You need games that can take advantage of 2 cards. COD is not one of them. I have the same card (maybe a different manufacturer; not sure whose you're getting) but my COD5 looks amazing! And I can play Crysis 3, one of - if not THE - most badass games where needing a strong video card goes, and it looks awesome. Would I like a second card for Crysis? Yes, but I'm not sure it will really get me all that much more. Probably not worth the extra coin at the moment.
Here's the good news - you can always add a second GeForce 660Ti in the future if you need/want to. By then, the price will have come down.
I'd go with one for now if all you're really interested in is a great COD experience. You'll get it in spades with the 660Ti