Good effort, but wont work. Users can easily put one device on WiFi, and another on cell towers.
Also, what about two players who legitimately play clean but want to play online in the same game and are both on WiFi.
I had a similar idea as Dominic but I came to the same conclusion as Matt. Practical Solution: Create an optional game filter to screen out games where there are players with duplicate IP addresses. This is not completely effective for the reasons Matt stated but it would make cheating in this manner require more effort which would presumably reduce the occurrence.