I also wonder that myself.
I know its a different ISP (Time Warner instead of Comcast) but my friend's kids have two Playstation 3's in the house, and they have both connected via a lan cable to her router/modem (its a combo unit), and they work just fine. They have two TV's and they all play the same game on both systems online without any problems.
So, not sure what the problem is, and why you have to have two different public IP's to be able to connect two xbox 360's in your own home.
Would it not function like having multiple desktops, laptops, or tablets connected to your internal network? Each one of them does not require their own public IP to function.
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